You're right, always two sides to a story - however, couple of facts that both sides seem to agree on:
They were asked prior to boarding to either change their t shirts or cover them up as Jet2 staff felt the slogan was offensive.
Once boarded some decided to take of jackets/cardies/hoodies etc therefore displaying the slogan.
End result, they were removed from flight.
Personally, I don't think it was terribly offensive and the Jet2 staff were a bit overly sensitive on behalf of the other passengers. That said, if check in/gate staff tell you to do something, then you do it. If you decide to disregard them, then you should be prepared to accept the consequences, which in this case was being chucked off the flight and about two grand in rebooking flights.
JF