Put simply, RMT will continue to use /tell and /shout even if the mail is opened up. It's easier to spam tells than it is to send mail.

All SE need to do is add a 500 gil or something charge for postage. That kind of charge is acceptable for individuals because we will only have to send mail occasionally, but RMT would have to sent up to 10,000 mails depending on server population. That would add up to 5 million gil and no RMT company is going to want to pay that because that is counter productive to them.