With respect to restraint, restraint is one of the codified principles that came in 1996 in Bill C-41. The principle effectively states that you don't impose a term of custody unless no other sanction is appropriate. It's effectively Parliament's direction to courts to use custody as the sanction of last resort. With respect to this particular context, the argument would be that you wouldn't put somebody in prison for stealing a car if some other sanction could do the job effectively.
The problem with a minimum penalty in general is that it prejudges, and some of the offenders who steal a car may not be the kind of people for whom prison is an absolute necessity. That's how it violates restraint.