It makes perfect sense. Money from subscriptions is good; extra money from ads is better. And as much as you or I dislike it, we're probably in a minority position here - if it was really cutting into earnings, they'd have cut it out by now.
Granted HBO and Netflix don't have commercials but I think they also both use product placement (edit: see granddaughter comment for more info) in their original content so it's not like there's some philosophical commitment they've made to not show subscribers ads to pad the bottom line.
$8: Hulu with x ads per show.
$9: Hulu ad free.
There was a True Blood/Apple image series atop reddit not too long ago but Googling reveals that HBO insists that they don't raise money for product placement but allow writers to use products so score one for HBO.
Humorously, Apple claims they don't pay for product placement but they do give devices away which is supposed to be different I guess.
Netflix ran into problems with Lillehammer due to Norwegian law against paid placement.
House of Cards with non-paid for Apple advert below:
That means if you were watching 2 episodes a day Hulu was making more from you in a month from ads then from the Hulu Plus subscription. And I think ads have noticeably increased since 2011. I guess we know why that autoplay next episode kicks in so fast. And I guess it goes to the true cost of these licensing deals and why Netflix has a much worse tv selection that's a year old.
[1] http://www.wirelessgoodness.com/2011/02/04/hulu-generates-0-...