The wording of the letter doesn't seem to be very enthusiastic either way
I agree. However, maybe it cannot be. Like I say, I am not a lawyer, but I suspect Unite has to be careful how it phrases any request to sign up. I would have thought that if it either seeks to coerce support (eg "sign this or we will kick you out of the Union"), or induce it (eg "sign this and you will get pay backdated to the time of imposition"), then BA could attack it and challenge the signatories' motivations for signing.
The letter looks as thought it was drafted by a lawyer. Everything else from the Union quoted on here is full of BA-bashing rhetoric, whereas the letter was very neutral. I just suspect there is a reason for that which is linked to the case itself.