The RLA puts up road blocks (cooling off periods, etc) to hinder that so that "the greater good" is not hindered. And the Union does it because it says, "sure you can leave your job, but if you try and come over here with your 747,767, MD80, DC-9, DC-10, 737 types and 18,000 hours you will start on the bottom at first year pay and benefits."
Unfortunately the government does put a hinder on the negotiating process when it adds restrictions and restraints to what a union can and cannot do - the biggest hinderance coming when a union threatens to strike. In a truly free market the union would have the opportunity to strike and thus have an important tool at its disposal when negotiating with management. Government involvement makes it very, very tough to do so. This is a very good reason why we should have
less government involvement (for those who favor government-mandated pay scales) in the industry and not more.
As for the second part of your post regarding first year pay, that too is in large part to what the union votes for. That could be fixed very easily. However, it would come at the expense of smaller increases in pay for everyone else. i.e., the company presents a sum of money and part of the union's job in negotiations is to determine how that money is divvied up.
First year pay could be increased, but few people are going to sacrifice on the mid-to-top end of the pay scale for that to happen.