The "No airdrops" proposal would really be better named "Threshold for airdrop proposals". If you want the threshold for other airdrop proposals to be high then you vote for it, and if you want the threshold to be low you don't vote for it.

You can want the threshold to be high in general but still support one proposal, in which case voting for both is not a contradiction.

This applies to the rest of the proposal system too (with the "return" proposal 0 being the threshold).