I'm not really sure why these new internet TV services charge based on the number of hours that they let you record.
Mostly (I surmise) because pricing strategies for differentiated (non-commodity) products are typically based on the customer's perceived value. i.e. Smart companies price things at the level they think people will pay for them. The underlying costs aren't really relevant except when they're too high to make it feasible to sell the product at all. Competition between companies affects perceived value of course (why pay $10 for ABC brand when XYZ brand is just as good and costs $8), as does availability of alternatives (that beer in your fridge at home does you no good sitting in the sun at a July afternoon baseball game).
You just answered your own question.it makes no sense to charge users different prices for different tiers of DVR storage. This is just free money for the streaming TV service.