Right, by giving everyone money, you are inviting spam.
However, by memo ONLY accepting twitter-banned people (using an automatic check) memo is using costly Twitter's labor to grow.
I estimate every person Twitter bans costs them $1 - $5, between lost revenue on false-positives, and manual labor of editors, and manual labor of computer programmers writing checks.
It's about to get worse with all the insane EU rules being rolled out. It'll probably start hemorrhaging money and need a bailout within the next couple years.