Who witnesses a Segregated Witness transaction and what incentive do they have to witness the extended block containing signatures?
Since exchanges and merchants won't accept the blocks, miners who mine invalid blocks won't get paid. A miner could take that risk if they wanted to, but it's no risk to the system as a whole. Only the longest chain that consists only of valid blocks is honored by non-miners.

I could imagine a miner trying to mine on top of a new block without first checking the extended block. But they'd almost certainly start checking the extended block and switch if it turned out to be invalid. This takes an acceptable risk of mining an invalid block while ensuring that nobody has any incentive to deliberately mine invalid blocks.