I think it's more a privacy issue, like CoinBase could start blocking PD withdrawals etc. Although I assume the "hot wallet" is actually just a collection of addresses that change all the time.
There is also no reason for a public cold wallet since there are no public investors risking money at PD (unlike those other sites that you mentioned.)
Pretty sure you can just get the IP of the node running the Bitcoind server to get a full list of addresses associated with PD. Or something along those lines... Know there's a way though