I have 1-2 +DLCs on Steam, but I'd gladly rebuy them DRM-free.

http://img3.wikia.nocookie.net/__cb20080529071824/cnc/images/8/81/Stealth_Tank.JPG

I own all three games but I would gladly rebuy them if they game to GOG.
Bioshock was the game that got me into gaming and now stands as my favourite work of fiction (and yes I have played System Shock 2, I still prefer Bioshock). Bioshock Infinite is definitely in my top 10. I still haven't played Bioshock 2 but I'm looking forward to it.

2K's DRM stance is infamous but considering that Bioshock 2 had scary-crazy DRM that kept me from buying it till last winter (now gone, thank God) but Infinite only had Steam it's possible that they're slowly learning.

I suspect it's a combination of the three: regional pricing, regional restrictions and DRM. In the video they mention the first two, but also give an example about publishers suggesting to change their policy against DRM, too, and just add a small serial number or something like that. It doesn't refer explicitly to T2, but it doesn't exclude them either.

Apart from the regional pricing, T2 has censored Bioshock 1 for the German market among others, so selling the uncensored version here openly for anyone and without any means of age verification is probably the last thing they want. Although during the discussions following GOG's change of policy in regional pricing, TET suggested that GOG might prefer to sell regionally restricted titles to not selling them at all. There was no word about whether this stance is affected by GOG partially returning to their old policy of "One World, One Price", so it's still a possibility that they could sell the game on GOG somewhen in the future while blocking German users from buying it. But if T2 was okay with selling their games DRM-free, they could already do that e.g. on GamersGate, where they could still believe their games protected by the GG client. And yet, as far as I know, the GG versions of Bioshock 1&2 still have their own DRM on them (possibly SecuRom? can't check now because, you know, I live in Germany :P).

I still seem to vividly remember, that Bioshock had the most hiddeous DRM upon release, up to a point where people had to call a hotline to unlock the CD-Key.

I cannot speak about GG since I don't use it, but I know this:
-Bioshock 1 had Securom as DRM, but due to the exorbitant number of complaints 2K removed it and now they distribute it DRM-free - at least, that is the case for physical copies. I have one of those on my desk right now (bought on Amazon, so the censorship can be easily circumvented)!
-Bioshock 2 used GFWL, but now has been "transferred" to Steam. I suppose we won't see it on GOG in this decade! :(

They are VERY stealthy.

GOG.com has something that fulfills that role.

http://www.gog.com/forum/general/gog_starting_to_agerestrict_game_purchases_now/post1

Yes, the GamersGate version of BioShock uses SecuROM.

Dang it! Oh well...

And how can they verify my age if I'm just using an e-mail address and a PayPal account for payment?

This is a rather old video and Capcom has joined GOG since, personally I would rather see some of Take2's (2K) old catalog as it has some interesting titles but those are probably used as bargaining leverage.

Probably the age that users set in ...

My Account-My Games-Settings

So Capcom changed their minds after they joined GOG?
How odd.

I'd buy them (yet again). I'd like to see the first System Shock here even more, though I understand it's in something of a rights hell.