When has such a policy ever been made explicit? As you say, the Batman vs. Predator comics fit seemlessly into the canon. BOTH Batman vs. Punisher crossovers were referenced elsewhere in Batman canon. Frank Miller's Spawn vs. Batman was canon to Spawn continuity. The Spider-Man/Batman crossover with Ra's al Ghul played into Spider-Man continuity of the time as well with Kingpin's wife. Hell, the Batman and Elmer Fudd comic is even canon to Tom King's run.
So clearly, crossovers aren't rendered invalid by mere virtue of them being crossovers.