It's the game publishers. I can't find the thread to link you, as it was a while ago, but Stardock explained it had to do with the game publishers. I do agree they need to keep putting pressure on them to get rid of regional restrictions. It's ridiculous.
As for Impulse itself, from the perspective of digital downloads, I like it a lot. But as I prefer no DRM and physical copies of my media, it's behind GoG(DRM free) and retail purchases (physical copies) in my list of preferences. Aside from needing it to update Stardock's games, I'll use it if I find a deal on a game I'm not terribly passionate about, but that about it (and sadly, they seem to be moving away from the really good deals they had going a while ago. But they still have the odd sale that's worth looking at).

As I understand it, it boils down to most big publishers having self-contained regional divisions, such as North America, Europe and Asia. These divisions are completely independent on an operational level and often have differing ideas about how products and services will positively effect their profit margins. For instance Sony Computer Entertainment America and Sony Computer Entertainment Europe have a long history of not getting on very well.
Regional restrictions are not a new problem, many games have been released at retail in the US for instance but never made it here to the UK, such as Final Fantasy Tactics and Xenogears on the Playstation, Katamari Damacy on the Playstation 2. Also a large number of sports titles and licensed products tend to be region limited.
The reasons for these decisions I would think lie with the cost of production not being justified by the projected sales, which in many ways is understandable (just how well will the latest baseball titles sell in most of Europe).
The problem I have with regional restrictions via digital distrubution is that these costs no longer exist so I don't believe that we as customers should suffer because large publishing corporations cannot get their act together and create an online sales division. That it simply the reason why I pretty much only use GOG for my downloadable game needs because until retailers start forcing publishers to drop this nonsense it will just continue.
The Regional Pricing issue is a little different and as far as I can tell is basically down to the greed of the publishers and what they can get away with charging. It is completely unfair and unreasonable and again like above I will not support it.

I do thank you both for the informative answers! I also should have said game publishers rather than game companies since the game creators themselves usually don't have much control over these things (My bad!).
Isn't it a bit ironic that divisions with the purpose to determine whether it's profitable or not to sell a game in a certain region, doesn't allow regional-free online? I mean, that certainly would be more profitable than restricting games to certain regions. But I suppose the divisions are afraid of losing their jobs or something.
Ah, how I long for a service that provides modern games at a non-regional pricing and with no regional restrictions. Is it really that much to ask for? Well, I registered and downloaded Impulse anyways, in hopes that at least the regional restrictions will change one day. And perhaps even regional pricing too! It does seem to be a nice service in any case.

If I was a cynical person I would hypothesise that Assasins Creed and Far Cry 2 have not sold well in the US. Therefore Ubisoft have offered Impulse the two games at the very low price of $19.99 each in an effort to stimulate sales. However both games sold very well in Europe so it makes more sense to stick with the $29.99 price point Steam currently has them set at.
I would have to be a very cynical bastard to think like that though.

Awesome Impulse update incoming:
http://frogboy.impulsedriven.net/article/363215

"Ready to Play supports any PC game. It will automatically pick up games on Impulse and soon from Games Explorer, Steam, and elsewhere."
great

Yessir. They have a video up here:
http://www.stardock.com/products/readytoplay/screenshots.asp

Damn, now my free SoaSE serial from CDAction is deemed invalid. Oh well... *uninstalls Impulse*

As I said in the related thread, it seems this deal wasn't okay'd by Stardock -- at least in regards to the keys being given away online. I guess if they had actually arranged it with Stardock, it was supposed to be in print media only.

Oh crap, yet another game-centered chat application? How many do we have now?
Steam
Games for Windows Live
Xfire
Ready to Play
...any others? How many do we actually need?
Someone desperately need to create a multi-protocol app for these. (Note: I'm NOT arguing wether one is better than another or which one has the most functions, just the idiocy of having multiple apps do the same basic thing)

You forgot Gamespy Comrade as well.

Impulse is the best. I love it a lot. I only have small complaints about it. Zoomba of Stardock demoed Phase IV of it at PAX for me, and it's going to be fantastic.

Impulse website and client updated.

Indeed.
Everyone check out the new version of Impulse!

I dont like the new theme. And, what does this ready to play do? I do like that finally the Impulse client is light as any other browser window, instead of the heavy bloated crap it was before.