Motleycrew answered your question about being fired. Probationary employees are at will. They can be fired for any reason, or should I say, without a reason. Manager having a bad day, fire a probationary employee and make it better. The Union can do nothing for them, thus the understanding. 
Where do I get my info, from the picket line. 
As for your friends who are not probationary employees, all I can say for them is that being scabs, they are going to have a rough time going forward. Their co-workers are on the picket line fighting for a living wage for them, and all county employees. To cross that picket line in an insult to everyone the work with. I'll keep it clean and say no more