Robinhood goes down to vendor street and into the shop of its preferred vendor, who is also the preferred vendor of most other brokerages. The vendor says "The street price on this item is $100, but we can get them for $95. We can get it to your customer for $97 and give you $1, or $98 and give you $2". Robinhood takes the $2, other brokerages don't, and Robinhood (crucially) lies about it, at which point the SEC gets mad.