The text isn't an order. A couple of my friends drive Uber in their spare time for cash, and they can work when ever they'd like. They just sign in to the app on their phone and they become available, and can sign out when they don't feel like working anymore.

They get texts when surge pricing is in effect like above to incentivize them to sign in and drive.

The point wasn't about whether it's advisory or an order, the point was that the actual price was not part of the message. Given that market principles are supposedly about using price signals, this is rather at odds with market principles, don't you think? ;-)