> Suppose you make an post in an internal politics discussion forum saying that you oppose the H1B program as it is, and then get fired because people claim that you hate immigrants and want your fellow coworkers to be deported. Do you think these unions would defend you?

In the real world case of long standing Teachers Unions in Australia (they vary by state) it is literally impossible to answer such a question on the basis of such a shallow construct.

The answer is both Yes and No - in any specific case the individual circumstances would be looked at - eg: as laid out Yes, they defend, however in most IRL cases the circumstances on the ground are far more complex, and it wouldn't be uncommon for a bunch of fellow peer union member coworkers to speak up in favour of not defending.