Because the left lane on 407 is often at 140-150 and to pass anyone you will have to go faster.
Also unless you suck at driving/riding, going 155km/h in a straight line with infinite visibility on a flat terrain with no hidden intersections is not dangerous. Again, unless you suck at operating the vehicle.
Not that I'd ever break the law. The law always makes perfectly 100% sense and we should all follow it. We should also raise the fines for going 50 over to involve capital punishment; this will prevent the speed demons from maiming and murdering innocent children that accidentally walk onto the 407, or the poor driver who just came to Canada and doesn't know how to use signals or check blindspots from turning into someone going 150km/h when they're doing 90km/h. We all know speed causes death!