There aren't enough people on this platform to attract enough bots to cause a level of spam that warrants a spam dm solution. Yes, I do get spam, but its occasional. If it can be a policy, and it doesen't need to be yet, it can be a simple solution to a problem that has plagued us in email today.
Zaps allow us to decide real value. If the operator of a bot really wants our attention in DMs, he can pay us for it. If we don't want to give our attention to someone unless they pay us 1000 sats, that is my decision. If I have some kind of misplaced ideology that dictates I should read all spam, I can set it to zero minimum. If its a real person, I can zap them back and it costs them nothing.