Jaynes believed that the only kind of probability was that caused by uncertainty about the true state of the world. So he was advocating MAXENT as a way to summarise one's (lack of) knowledge about some system.

In fact, this point of view probably made him less likely to accept a probabilistic theory of fundamental physics. If you've discovered how a deterministic universe can appear random, and how probabilities can still be useful for measuring one's ignorance, then it's reasonable to suspect that these are the only kinds of probabilities and that the universe really is deterministic.