HBO is owned by Time Warner who are trying desperately to maintain the relevance of cable television along side Internet service and separate and individually valuable items. They're full of it, of course, and they will see their model collapse under its own weight (almost certainly because more agile competition will come a long and eat their lunch), but until that happens, they will continue to attempt to incentivize subscriptions to their service (and by that, I mean cable TV, not HBO).
Funny thing is, it does work. HBO routinely has better programming available than the vast majority of television specifically because that margin of quality is their marketing pitch for subscribers. ABC doesn't have to take any chances because producing mediocre content will continue to trap a third of the people who are too lazy to figure out that there are non-network sources of entertainment.
Fortunately, that model works just as well with any voluntary subscriber model, even one that's not nearly as abusive to customer budgets as pay-TV.