Prepositional phrases (except of) were always illegal when describing something like Things that are... for example. They were acceptable when describing phrases like Why you... or What a _____ might say.
Right, I'm aware of that and take no issues with it. What I don't
understand is why Mike gives the blanket instruction, "No prepositions," when that's clearly not
the case. Winner's circle clues have included words that are prepositions and have not been buzzed (take last night for example--"for" ((which is a preposition and a conjunction)) was included in the celebrity's clue that won the contestant $20,000). Even for Mike to say "No prepositional phrases" would not be accurate since there are indeed occasions where they are acceptable, which you pointed out.
Don't get me wrong. I love the show overall, and I think Mike is doing a very good job so far, and it's probably not even he who came up with the wording of the rules. I just don't get why, on this version, they felt the need to say "no prepositions," when none of the previous versions have, especially since it's not true in a good number of categories we see.
And yes, for everyone who's thinking "this is a miniscule point in the grand scheme of things," I agree; but that's what some of us like to do here--discuss these little intricacies that come up from time to time.