My impression was that "lending" digital copies this way is less likely to upset publishers, since it's more or less analogous to the already-acceptable practic

My impression was that "lending" digital copies this way is less likely to upset publishers, since it's more or less analogous to the already-acceptable practice of checking out physical books from a library. Allowing an arbitrary number of simultaneous readers would take better advantage of the digital format, but is much more likely to attract a lawsuit.