I don't believe it is simple, & I admit to having very little insight into what "exactly" they could react against.
I could imagine it would have to be an objection to having the crew establishment, and of course the body of pilots available to participate in union votes, diluted by an excess of non easy employed contractors, or some similar objection.
It is a situation that was avoided in Ryanair courtesy of the non union recognised status of the company, and it is a big pill for BALPA and the Easy employees to swallow, but swallow they must, or they will choke on something much bigger in the future.
I agree, the Cadets can do nothing, I don't say they are angels, but they know if they refuse, the next guy in line will say Yes.
Merde alors ! !