This is wrong on several levels. First of all limit orders place a limit on the worst case price that can be executed but SEC rules impose a duty to execute orders at the best price. If there's a $100 buy order and the best price is 99 dollars, then there is a duty to fill the order at 99 dollars.

Second, the allegation made by the SEC, for which they most likely have very strong evidence, is that Robinhood didn't fulfill its duty to execute orders at the best price to the tune of some 30 million dollars.

Not a market order which majority of people on Robinhood are using market orders since they are the default. Most don't even know what a limit order is I'm guessing.

The default trade option on Robinhood isn’t a dollar value buy order. It’s a number of shares order at the market rate.