you can stop working at any time, so that's what they do, and then start working for another company. Any employer can't have a problem with working a second job, so long as you're not violating a non-compete.
Pretty tough to have a non-compete when you're casual and/or a contractor. Some jurisidctions make this illegal in even more of an employer/employee relationship, most aren't going to enforce it in an Uber-style job, and it would never be cost-effective for Uber et al. to go after a contractor. They would just kick the "offender" off the platform - which might still be very bad for them.