I don't like/didn't like this practice from DD (I think they're finally phasing it out after much criticism), but it worked exactly the same way tipping at restaurants works, so that is a precedent they can call upon as well.
The federal minimum wage is set to $7.25 per hour, with a very clear exemption for tipped workers, that the employer contribution only needs to be $2.13 per hour as long as the tips bring the total wage up over minimum on a weekly basis. If the employee gets lower tips, the employer is required to pay additional wages to bring their total wages up to the guaranteed minimum. See: https://en.wikipedia.org/wiki/Tipped_wage
The problem isn't that DoorDash is doing some new, nefarious thing. They're doing exactly the same thing a restaurant does when they pay a server somewhere between $2.13 and $7.25 per hour depending on how customers tip. The problem is that apparently most people don't understand how tipping and wages for tipped employees have worked for traditional restaurants for decades.
Because if that were allowed an employer could hire you at $20/hour (for example), and then decide at the end of your day to change your pay rate to $10/hour and that would be ok - because by your definition that isn’t wage theft.