Technically, if the employer in question is the government...
Still, it is a bit misleading to say the "dues are collected by the government".
I suppose I mainly object to the idea of dues being collected by the employer, from an employee's paycheck. It ties in with my dislike for closed shops.
For example, I did a google search for "teacher protected by union", and it came up with the following, among many others:
These two appear to be opposed to unions protecting bad teachers:
Dirty dozen: 12 bad teachers protected by tenure and unions | Fox News
Unions protect bad teachers, not students : The Daily Collegian
And this appears to support unions, countering the idea that they protect bad teachers:
Six Reasons Teachers Unions Are Good for Kids
Edit: I should note that I have not read any of those articles, due to lack of time at the moment.