Liberals of today are quite literally the Progressive Conservatives of yesteryear, before Preston Manning and the Reform party took "Progressive" out of their ethos by merging the Reform Party / Canadian Alliance Party into it and thus hijacking their mandate.

So really there's no irony about it. The harder right Liberals crossed over to the new Conservative party to play in Stephen Harper's sandbox. The old-guard "Progressive Conservatives" who were not comfortable moving more right moved over to the liberal party which ended up moving a little bit to the right as a result.

Implementing and enforcing a rule like that would punish the constituents of the riding. If the principle is that an MP should first and foremost do what's best for his or her riding, then joining a party is far superior to sitting as an independent. It gives them a seat in caucus where decisions are made, input into policy positions, a chance to chair committees and potential cabinet or secretary roles. A party MP can walk into ministers offices and lobby internally for local projects. Independents don't get these benefits. This ignores all the other benefits like campaign funding, etc. For a riding as influential as Markham/Unionville this is a substantial gain for the Liberals and they will no doubt be willing to support and listen to the constituents, and treat them quite favourably.

The Canadian body politic should have learned and should recognize that we vote for MPs, not specifically for parties. The parties are political associations, not constitutional actors. If you want the best results for your riding, you probably don't want an independent. Sitting with a party maximizes impact, sitting as an independent maximizes moral autonomy.