Payment for order flow is not front running. The PFOF trader only gets right of first refusal on every trade; whether they take it or not, the broker is still obliged to execute the trade at NBBO or better.

It actually gets retail investors better prices than institutional or professional ones, since the counterparty can safely assume that there's "dumb money" on the other side of the trade.

That's not to say it's not controversial in some aspects, but it's not as simple a situation as you make it out to be.

Also, somewhat ironically, the front running risk on many decentralized exchanges is much higher due to MEV being a hard problem to solve trustlessly.