It could be a euphemism, but a more charitable explanation is that they are trying to acknowledge that their rules have a lot of gray area. "Guidelines" could be interpreted as a rule, but a specific type of rule, one that requires more of a judgment call to determine whether it was violated.

Ideally rules would be clear and objective. Maybe there are cases where Apple could be better about that, but it's not always possible. For example, one of the rules is to select the appropriate category. I can't imagine how you'd write a rule that perfectly defines proper categorization. Another example is that apps must "use power efficiently", but good luck completely defining what is too wasteful or what is/isn't a worthwhile use of battery power.