As usual , when UK meets France, there is a lot of emotions floating around.
At last, there is grounds on the european law since 2006, which does apply to all businesses, not only aviation related ones ( the most restrictive being the truck drivers one)
It is very simple : either the foreign company has an operating place in the country or not. For this case, MRS was a base, people ( crews) were having their rest at this base, they had lockers under their names, etc.. therefore, according to the law, the company should have complied with the obligations of this country, no more, no less. This was including to pay social security and so on in this said country. Since they failed and got caught they have a fine for having been not compliant.
Since January 2012, the european law is in effect, and after a transition period of 10 years, EVERY foreign company that has an establishment in France ( or any other foreign european state) have to comply with the same rule. A small change here, if you have a contract, and if this contract is amended for whatever reason during the transition period, you ( company + employee )start right away to pay these taxes.
Precision here also ; taxes have a different meaning depending the countries.
In France for exemple, you are not taxed at source, meaning that after the deduction from gross to net, the employee still have to pay the income tax.
Netjets case is somewhat different, but in its essence of EU-OPS now EASA, it is quite clear that the company has to nominate a base for every crew where the said crew will start his/her duty from. In the unique Netjets scheme of "gateways", this is where the crew leaves ( or where he/she has its center of interests read family). Proactivity made the rest and the call was made.
When the crew has no life, when IT ( there is no more individuals there, just a necessary burden) can be shuffled around europe every 4/6 months or so, it is more complicated to get a fault on the company, but at the end of the day it is the same law; and in this case it is found under all the new double taxation treaties, the taxes shall be paid where the passport of the individual is issued.
Now the talks can go to an endless path, but this is the law, like the yellow vest in UK....or CAP 731 ( not compliant with ICAO), or else.
EZ did its homework and is now number one airline out of Nice airport...
Enjoy your flights, because this is the only thing that you will have left at the end of the day; in an airline, airbone crews salaries are accounting for a very small percentage of the fare cost; do not shoot the messenger...