My take on this is the following:
It is a change in personal tax legislation in Germany which was obviously done in order to close a loophole that many crews, including easyJet crews, were able to take advantage of. Taking advantage of this loophole was perfectly legal and you guys seemed to have got away with paying very little tax for a long time - Well Done!!!!
I don't have much knowledge of easyJet but do believe that you guys have ultra high expectations if you think that they should be advising you on taxes - that sort of thing wouldn't happen in the vast majority of companies unless they were regularly moving people earning $1m+ around the world.
Most companies based in the UK wouldn't know anything that was happening in German personal tax legislation (in fact having spoken to a couple of my colleagues who specialise in German personal tax don't know about it (most governments don't like telling people about tax changes unless they are beneficial!)). Not sure how eJ would have found out about it but I do know they have spent lots of money looking at their own tax position across Europe in the last few months so it could have come out of that.
At the end of the day people have to pay tax unless they can afford to spend stupid amounts of money with the people that I work for, and take the risk of annoying the Authorities. If you thought that what you had would last then you were simply be niave. But nevermind - you now get the chance of claiming lots of spurious tax deductions in Germany and somehow getting away with it (which is how things happen there). I seriously advise talking to a German aviation specialist to work out what you can and can't claim - you never know, it may even save you money!