In finance, nothing is illegal if the profits outweigh the fines.
Citaldel paid handsomely for order-flow information from Robinhood. They made a lot of money off retail traders. They paid a fine IIRC equivalent to a few day's profits.
If you are curious about a brokers position on PFOF you can look up their disclosures. SEC Rule 605, 606 and 615 are the search terms you want when looking these up. Fidelity has a similar disclosure on this as Vsnguard, which is that they don’t engage in PFOF except for some options markets.
Robinhood got in trouble for false advertising about PFOF not because they engaged in it, because again, PFOF is not front running and not illegal.
PFOF isn't (typically) illegal, a better word might be "controversial". There's nothing free in this life: the zero commission brokers are making it up somehow.
While the studies on how PFOF effects execution quality are varied, this summary [1] from Wharton seems fairly balanced. It's not as simple as citing NBBO and moving on.
Personally I'm suspicious of the practice mostly because of the pretty clear conflicts of interest that it creates. Again, this is controversial, but the people arguing it's ok are for the most part making money from it.
PFOF is not front running. Market making is not front running. Full stop. This fact is only controversial if you fundamentally misunderstand what market makers do.
Citaldel paid handsomely for order-flow information from Robinhood. They made a lot of money off retail traders. They paid a fine IIRC equivalent to a few day's profits.