The issue is of public perception. The disputes may not be related, but the perception of it (and even the Mail, Express and Telegraph have made it clear that they aren't related in their coverage, it's not down to the press) publically, even if people understand the situation, is that this has been ASLEF overplaying their hand massively. As I've said it's almost certainly an hubristic cock-up but that doesn't alter how it's seen by a large number of people, who don't care what the dispute is about, all they're interested in is a union announcing more strikes just after receiving a very good pay settlement when they thought that they were about to get a more stable rail service.
It's now up to ASLEF to try to justify it to the travelling public. I suspect they're going to struggle, however justified the action may be.
Worth noting that Labour banning it would have no effect on the specific circumstances of P&O fire/rehire.