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.
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...