that it's just as risky and lacking in airmanship for a large aircraft to fly over a glider site at or just above the top of the launch envelope without knowing where the gliders are flying.
How do you know the pilots hadn't briefed themselves on their route and knew what they were overflying. Was the Easyjet in contact with ATC? Did ATC pass this local information to the pilot?
It sounds like you are presuming that this Easyjet a/c was bumbling around class G on it's way to Luton, and not talking to anyone.