Assuming for the moment that the Grits knew Stronach was waivering, their strategy of refusing to acknowledge confidence votes makes perfect sense.
If anything it makes the entire matter even more unprincipled; but that cannot matter to a Dithers desperate to stay in power. The only thing which counts from the Grit perspective is staying in power and, that goal firmly in mind, it simply didn't matter if they had to trash constitutional convention. After all, they were pretty sure that with another week Stronach would come across.
For those of us who believe that the Liberal Party of Canada has become, more or less, a mafia, this behaviour comes as no surprise. The only surprise is just how damn good at it they are.
Update: Colby Cosh invoke Michael Bliss to make much the same point at somewhat greater length...