With the per-delivery model, the drivers have the option to pick and choose which jobs to accept.

Being speculative, I believe the scare tactic being used is: the driver can be assigned very unattractive deliveries without the power to refuse. As someone that does not do deliveries for any of these companies and periodically viewed posts from /r/doordash, I can only guess that this will hurt a smaller percentage of drivers that formulate a metagame to maximize their delivery income.