Drivers were forced to watch political ads before they could start working. They were forced to read pro-prop-22 messages before they could start working. Some of these messages warned (some might say threatened) that the drivers would lose their jobs if Prop 22 passed.
Forced to watch an ad != forced to vote a certain way.
And again, a threat implies it is done maliciously. You think it would be better if the companies just stayed quiet, and then just out of blue one day said "sorry we're shutting down operations, you're all out of jobs"?
They get coerced to watch ads, not coerced to vote a certain way or have a certain opinion. The reason they started supporting prop 22 after watching the ad is that prop 22 gave them minimum wage and healthcare without being employees, which is exactly what most drivers wants.
https://en.wiktionary.org/wiki/if_it_looks_like_a_duck,_swim...