I wonder if it's the MOs they care about, or just the transferring money in and out instead of investing it (and maybe they'd also shut people down for doing ACHs in and out, without MOs being involved; I vaguely remember that brokerages don't like a lot of transfers in and out).

I've been depositing smaller amounts of MOs to Schwab and sometimes investing that money. Though, that doesn't scale to large amounts (unless you happen to want to invest a large amount).