Of course it has nothing to do with shorting in particular, except that shorting (at least until the Winkelvoss ETF gets approved) requires using precisely the same Bitcoin financial ecosystem that people thinking it'll crash are unlikely to have much faith in at all.

What you say of the exchange business model not being exposed to price movements in principle might be true of properly capitalised and regulated exchanges, but its less evidently true of the exchanges that actually exist.

The most recommended place to short Bitcoin is/was Bitfinex...