Hardforks are orthogonal with miners. If a miner is not complying with the rules of the network, then as far as the network is concerned they simply aren't miners. It was "classic"'s choice to gate their hardfork on miner support-- perhaps not a bad choice (though 75% is pretty much the worst possible threshold)-- but no force of nature made them do that.

The best reason for classic to use mining support as the trigger is, I believe, because most of the support for it is substantially fabrication and they believe it will be easy to trick the small number of miners needed to reach 75%, and by taking away 3/4 of the network hash-power they hope to coerce the users of Bitcoin to follow along. I think they greatly underestimate miners and the users of Bitcoin.