You might ask that, as some in the NDP/left blogosphere have, and I think it boils down to 2 words: realism, and protection.
First the realism part. The NDP has a base of support in the Toronto area. If they refused to support this back-to-work legislation for an intensely unpopular strike before it had even started, the Liberals would remind EVERYONE of that in the next election - not that the voters would need much reminding, and I really think they knew they would have paid an electoral price.
Secondly, there are some people who I’ve talked with who feel doing this action will protect the NDP from alienating Adam Giambrone, current head of the TTC and potential provincial NDP future leader, or at least star NDP provincial candidate, from lasting political damage and tarnishing his star - at least with the NDP.
Faced with both scenarios, the NDP had no choice but to support the legislation.
While Giambrone is a dedicated Dipper, it made total sense for the NDP to do support him over the unions in this.
Some in the NDP think that Giambrone is a future star for them, and he think that he is taking a principled stance in this labour negotiation.
I’m sure the NDP would love to have Giambrone run for them in the next election. If so, they can’t alienate him by taking the side of the unions. The NDP can be sure of trade union support in the next election, but there is no question that they will harm their chances of getting Giambrone to run for them if they supported this very unpopular strike by the TTC union.