Firing someone for joining a union is against the law. In fact firing an employee after a grace period is difficult in Scandinavia unless they are clearly incompetent or criminal.
For that reason it might even be that Uber doesn't worry too much about high turnover because it means that most of their workers are sackable without having to give reasons or compensation.
They work around that by calling these employees 'freelancers'. Which is of course bullshit since there's a top-down relationship, but these employees don't have the means (resources/info/money) to combat this in a court of law.
The trick abused before was hiring people as interim, and then sacking them right before they had to be hired permanently by law. I know first hand even the government in The Netherlands did this in '00. However this still means these people need to get minimum wage (actually, more, since the employment agency also demands a cut). As you can see, the freelance trick is more lucrative.
For that reason it might even be that Uber doesn't worry too much about high turnover because it means that most of their workers are sackable without having to give reasons or compensation.