Just being blunt, but if you got pissed off at your aunt like that, I would call you an ungrateful spoiled brat. Doesn’t matter how much you paid to move your stuff in. I don’t actually think you’re a spoiled brat, because it’s just an analogy and expecting something from Posterous as you do is at a much lower level of entitlement than expecting something from someone letting you live with them rent-free. I see your point of view regarding Posterous and I can respect it although I disagree.
IANAL, but I think the only reason that a website offers terms of service (beyond any minimum legal requirements they must live up to whether they like it or not) is to give the appearance that they’re nice guys that care about their users. That makes users feel good; it’s a PR decision. But the business doesn’t have any obligation to make its users feel all fuzzy inside about the service. It’s just that it’s usually in the best interests of the business to do so, to the most reasonable degree possible.
And it’s good that business don’t have this obligation, because users can be stingy and irrational. Like, for example, when they get unjustifiably angry when the business finds a completely harmless way to support the free service.