This is pretty simple. If serving in the US military you end up with orders that benefit a private company, that company should pay the difference that person would have been paid if hired by that company.
I bet no one joined the Coast Guard thinking they would be cleaning up oil spilled by a private company's broken pipe. I bet no one joined the Army to drive around (provide security) for rig workers of an oil company.
Companies pay huge premiums for people to risk their personal safety while operating over seas, often well over $100,000 a year. The fact a Vet can be ordered to assist in a company's private, for profit, operations while making SIGNIFICANTLY less is abhorrent.
Make it a law. When Private companies use Military personnel, PAY THEM FAIRLY FOR WHAT THEY DO!!!