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.