I guess if you disable z2t by default, like tromp just said, they would kind of be forced to do allow depositing / withdrawing 'private coins'.
Or they would just delist Zcash from this exchanges, and they don't have to explain why.
That's actually what Australia did, according to my employer (who is incorporated there).
See, lawmakers do not care about "normal address" and "private address" - they are both random strings of text to them, without a name and postal address.