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.
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.
[1] https://wifpr.wharton.upenn.edu/uncategorized/research-spotl...
Edit: No, it's not in the US at least. It mostly just allows the broker to internalize orders if they prefer.
I consider most financial institutions to be criminal because it is always their clear intention to circumvent the spirit of the law as closely as possible. The intention is to reap the benefits of breaking the law, without the risk of consequence. When the pitchforks come, these are going to be the criminals being chased down the street.
By the same token, I do not consider a parent who writes a bad check for groceries to be a criminal.
If you have a legal route to $1M and an illegal route to $1.5M, the rational calculation for fines is against the $0.5M delta, not the full amount.
