They DO require this. It's just that the GMs are so slow to lock suspicious accounts, that the RMT bots can do a lot of advertising before the paid account becomes inaccessible. So it's like a $15 investment that gets them a month of advertising.

If the accounts were checked immediately after receiving reports for RMT activity, and the guilty accounts were locked pending review prior to account termination, then the cost of using paid accounts would be too high, and RMT would stop with all of the relentless spam. They would be forced to use less overt methods of advertisement.

The problem is entirely that SE takes too long to take action, since they prefer mass bans once per month. Buy an account, harass people for a month. Why bother having faster response when you can get paid each month to let spammers run wild?