Alright, first off - does Portal 2 REALLY have Day 1 DLC (ie: significant parts of game content - like Shale from Dragon Age - locked out unless you pay £15 or use a one-use-only code to unlock) or is it just that there's a load of alternate skins available to buy?
Secondly, I know exactly why companies are doing this, and I know squarely where the blame lies - with you. Specifically, with every one of you who'd rather save £5 by buying used than paying for a new game.
Seriously, I remember going to buy Dragon Age on the day of release. I went into GAME (which I'm convinced is the UK branch of Gamestop, and have thus stopped buying from them) and selected out a £40 "New" copy of Dragon Age, complete with the free unlock codes to get me an extra £15 - £20 worth of stuff.
Do you know what the **** behind the counter said? "We have a used copy for £2 less." The company had gone and given some moron £20 for a brand new copy of Dragon Age, which they were going to flog to me for £38, and thus STEAL FROM ME by making me pay £20 online for content I get for free buying new. Incidentally, how much do you think the game developers get for that used copy? Nothing.
THAT is why Day 1 DLC exists - because businesses like Gamestop are built around taking money away from the game developers.
If you don't buy new, you don't have the right to protest Day 1 DLC. If you do buy new, it doesn't effect you anyway.