I doubt that liability is much more of an excuse- how often do disgruntled candidates actually sue corporations, in any context? If a company doesn't want a candidate, there's not much value lost by them burning bridges with them, by refusing to send them feedback, or even a rejection notice. This is especially true for big companies who receive a lot of candidates. And for smaller startups, they simply lack the time and energy to give detailed responses to people they pass on. A failed candidate is of marginal use to a company.