It's not about being owed anything, or some moral or ethical obligation. It's about the fact that D&D players, even in the broadest sense, are a dwindling and disenchanted customer base, and WotC, if they want this edition to do better than the last one, would be well served to use every tool at their disposal to garner the interest of any potential players, including using the playtest as a promotional tool with frequent and substantive releases. Even if they're releasing poor quality material, it's just a playtest, right? (And, not to put too fine a point on it, but where is there to go but up?). So the bottom line is not that WotC is obligated to do anything, merely that some of us stakeholders think they should do something.
And if I'm WotC, potential customers are as interesting to me as actual customers, because I need them both to buy my new game or might I lose my job. One need not be a current paying customer to make meaningful judgments about a company.