I think there's a big difference between probation and training.

I have never heard of anyone taking mil leave in training and it wouldn't suprise me if the company had some recourse in that case.

Probation on the other hand is still protected by Userra. They might be able to fire you for no reason...as long as the government doesn't think it's because you were a reservist/guardsman. I don't think ANY airline wants to get anywhere close to that line.

My experience is that initial training pretty much trumps everything, but probation doesn't.

I also think that as long as you were abiding by the letter of the law (USERRA), you're very good shape. I don't remember ever hearing of an airline fire a guy for mil leave that was IAW USERRA.