Just because someone freely accepts work, doesn't mean its automatically free of exploitation. Exploitation just means one party is taking advantage of another parties weakness.
Rideshare specifically - I don't think this exploitation is unilateral, but it does exist.
Example - UAE commercial builders freely attracting Pakistan/India laborers and then holding their passports hostage when they realize it's not worth it. https://www.rt.com/news/353432-uae-migrant-workers-slavery/
EDIT: I should clarify - there are degrees of exploitation, the UAE example is an extreme (but used as an example because its so clear cut). My overall point has more to do with people "freely choosing to work" as an idea that people can't be exploited as a result.