(If you don’t understand why they could do that, c.f.
kalzumeus.com/2019/6/26/how-… )
I think there is a little nuance, honestly. Cutting commissions encourages trading but trading is, for the overwhelming majority of their customers, bad.
Commissions are also visible and legible; the actual way they make their money is far less so.
*sigh* No, they are encouraging customers to keep (too much) cash deposited with them and then buying lots of mortgage-backed securities.
This drives product decisions, like cash allocation on their roboadvisor.