The answer is simple, maximize profits. It's what they are all about, obviously. They use corporate tactics to sell their games (the awards, big hype, opening party for the celebs so the sheep of the world will follow, etc), so their motives are quite simply... money.
Personally... I hate myself for buying them, it just turns my stomach really to support this company. But I am kind of OCD about things and want to add it to my Fallout collection. I don't see the point in DLC really unless your ADHD and want it now, now, NOW. I prefer to have everything on DVD or CD so you can just get it and play it later when you feel the need down the road. I don't sell any games, ever. I buy lots of games when I am away on contracts (my ex-gf's collect them in their mail and hold on to them for me till I get back) and play them all when I get back half a year to a year later... then exercise for a month to lose the weight I gained gaming and back I go... staying longer now due to being stabbed in the knee THREE times (no you really don't pass out from the pain, it just hurts ALLOT and you wish you would pass out), but its good as there are more games now than usual for me to catch up on. I would probably feel allot better about Bethesda if they released all the real (origional meaning Interplay) Fallout 3 info and files for us older gamers to have our fun... but that isn't economically viable so not happening.