All the above. Most companies only pay what they have to, in order to hire qualified workers. As long as pilots are willing to work for low wages, low pay will continue, and I see no trend toward holding out for more money. Indeed, there is an endless supply of low-time pilots willing to work for little or no pay. The oil companies are making so much money that they have to spend a lot of it, and they don't care what the helicopters cost, so there is plenty of money for everyone. A couple of years ago we discovered that one of the major oil companies doesn't even bother to break out helicopter expenses as a separate item in their budget, it's just a miscellaneous expense, paid from petty cash as it were, even though they were paying ~$1 million per month. When your net profit is on the order of $100 billion per year, a few quid per pilot per month is negligible.
Having done both jobs, I have to say there is no other explanation for the difference in pay. Offshore, you have more capable equipment and two pilots. IFR is easier than VFR when the weather is less than perfect. In US EMS, you're doing night approaches to unprepared accident scenes single-pilot, in marginally powered aircraft, but most of your time is spent watching TV or reading PPRuNe, and you only have to do it a couple of hours per week, so I guess it all averages out somehow.