I read over the FAQ on Check 21, and it doesn't say anything about this practice. The goal of Check 21, as I understand it, was to make things more efficient by allowing banks to scan paper checks and then pass the scans around electronically, thus accelerating the process and cutting the costs of check handling. There's no reason why a bank has to take the money out of your account when you write the check. The money should stay there until the other side claims it and then it's transferred.

With electronic payees (as for all my major utility bills and such), there's very little latency for this, so it doesn't matter. For checks that are printed and stuffed into envelopes, that latency is a much bigger deal.