In Canada I have received discounts for cash, and I've seen it advertised, so there doesn't appear to be a problem. However, it's still rare, and you have to ask yourself why it is rare. In Europe it's rare. The argument has to be that it's rare because cash is costly, and therefore you don't want to put people.... In some places it's used, and for certain transactions as well.
Mr. Mulcair worries about avoidance of income tax. Cash is one of the clear ways of avoiding income tax. If you want to pay for something and avoid income tax, you pay in cash. That becomes a problem, and retailers may want to steer you to cash exactly for that reason.