There are a lot of things that exchanges could do, but don't do. If people had addresses that would have gained CLAM, then they exchanges could have given them out. They could link wallets to Byteball and give out those too. There are lots of airdrops and now we will have a forked alt coin too.
I think the safest bet for an exchange is to say that they won't give out any of the 'bonus' coins, if you want them, then you have to keep them in your wallet.
Include sites wallets too. Actually, I'm surprised that more haven't taken the Coinbase stance that won't give any options to users to access alternative coins. I don't know any site or exchange that has ever given out "airdrops", but many have now promised to support alternative coins should the fork(s) happen. I suppose it's the ethical thing to do, not that that has ever come into play.