Interesting idea, but I don't think that's the problem. If somebody sent steem to the 'poloniex' account where the memo field didn't match one in their database, they could easily return it to the Steem address from which it came, minus a handling fee if necessary (which could be explained in the memo field of the return transaction).

I think it's more likely that due to the resources and skills required to run a Steem RPC node, they have some difficulty doing so, or integrating it with their systems. Maybe since Bittrex and Blocktrades are more integral to the Steem ecosystem, they have more expertise in running the necessary servers.

This theory could also be wrong though.