I have always wondered what use is this with regards to spam and sneaky address lists. What exactly prevents someone from cleaning up the addresses before letti

I have always wondered what use is this with regards to spam and sneaky address lists. What exactly prevents someone from cleaning up the addresses before letting them out? If I were a bad behaved site and sell (sold? sorry, tired and non native speaker) my userlist, i'd probably remove all plus-suffixes from the addresses. Same as if I were a blackhat stealing them to be sold. Honest question, no snark.