Don't blame the US for this one. Sure, Canada could mandate that. Would GM file a lawsuit? Probably. Would they WIN a lawsuit? That is not at all for certain.
If all the regulations changes that Canada mandated were backed up by supportable, repeatable studies - not just one single study that was done 5 years ago in Europe - the chances of a restraint of trade lawsuit prevailing would be far lower than you make it out to be. Not to mention the political implications being extremely serious. Imagine for example if GM were to file this lawsuit and Chrysler were to file a brief supporting Canada then start advertising that their vehicles are safer than GM vehicles. Get a few others involved like Ralph Nader and when GM sees their sales affected, that lawsuit would get settled out of court quicker than grapes through a goose.
You have to understand that in the US, the THREAT of a lawsuit is used much more than the actual lawsuit is. Just about everyone will claim they are going to sue your ass if you do something they don't like - but if you go ahead and do it anyway, most people won't end up filing. This is how the US culture reacts to things it doesen't like - a lot of bluster and talk, very little actual doing anything except for trying to propagandize the mob in the street.
If Canada really wanted Amber turn signals all they have to do is mandate those plus about 5 other safety improvements. If GM sues, then they settle out of court and agree to only mandate the Amber turn signals. Then 5 years later Canada does the same thing again, and settles out of court again. Little by little all the stuff that Canada wanted would eventually get implemented, because as you pointed out all the automakers would have to do is change the US regulations to match, to have a single inventory. And since the US regulators are basically in the pockets of the automakers, that wouldn't be too difficult.
That is how politics/business works in the United States. Sorry that you don't like it, but once you understand it, it is quite possible to get things done. Many other people have got a lot more serious safety improvements forced into law down here than amber turn signals, just by playing the game. If Canada regulators don't want to play the game then they only have themselves to blame for it.
Ted