Doesn't this feel predatory though? They already charge a transaction fee? Any reputable online-brokerage that charged above market prices for stock market orders would get destroyed for this.

Lots of online brokerages do this. This is how almost every Forex brokerage operates - you buy above and sell below market rate, which is how they make their money.

Only the bad forex brokerages. The good ones (ECN) take commission instead.

Ahem use gdax, it's 0.25% transaction fees. Coinbase started when people were buying btc here and there once a year and forgetting about it, the main benefit or their recent expansion is the ability to freely send money to and from coinbase to gdax and then trade with 1/8th of the transaction fees and automate with their gdax API...

If you are doing high frequency trading with coinbase and not gdax or any cashing out with the fee you are doing it wrong.

They offer a product with lower fees for those who qualify. Others say this is standard for a brokerage. Seems like a fee for additional risk in serving less-vetted customers.