Your assumption on the work area is correct, but nothing else changes the difference between being a full time employee of a company that pays its share of SS, workmans comp, etc and being a contractor ($500/day for Airlog, etc.) The contractor is responsible for so much more then just the taxes that all must pay. The individual doing contract work in the GoM has no chance of being tax exempt, the individual working overseas has that chance, but not all understand the IRS rules very well.
As a full time employee I do receive credit for SSA and fairly decent (not the best by any means) benefits. I am not saying that Evergreen is an awesome company to work for, but it does beat the contract work in the GoM in pay and benefits. (At least for my situation. A semi-retired guy who only wants to work a few months a year could be better off on contract, but thats the only way it makes sense to me.)
BTW, no matter where you work unless you own the company, you are a temporary employee. (Even the union shop people can be terminated, not quite as easy, but they can nonetheless.)