I don't see either approach being morally wrong (granting or denying to employees contract veto power), they're both just different ways of governing an organization. These seem like the underlying questions however, and it seems to me that those calling to end the ICE contract should be prepared to answer them.
To use a metaphor, Alice: give me a dollar; Bob: OK here's a dollar; Alice: Thanks but now I need five dollars; Bob: are you just going to ask me for $10 next? Where does this end? Alice: Aha! That's a slippery slope fallacy.
Slippery slope says “if you do A then it will lead to B and eventually Z.” At the moment, we’re not at “A” we’re already at “B,” so I think asking about the rest of the sequence is reasonable.
If management is making concessions to satisfy the workforce's sense of ethics, I think it's reasonable for management to say "OK, after this concession will your sense of ethics be satisfied for the time being? Or is this part of a larger package of demands, and if so what are they?"
To ask a party making concessions to not consider the context at all, the lead up or the implied consequences ("if you did X you'd be a hypocrite to not also do Y") is unreasonable.