They used to because the Tab tiers were fixed at a dollar amount that got paid off at a percentage of your bill. No matter how much your Tab was, your bill was basically just your plan + $5 if you wanted Tab M (or $10 for Tab L). So they had to change retail prices to make something go on sale.
Nowadays, it's still tied to the retail price, but the Tab payment system has changed to being charged separately from your plan. The problem with changing retail prices below the $360 mark is that it creates unfair bills.
Let's say Bob got the Moto X Play with the new Tab system last month and paid the regular price at the time, $50 upfront ($410 - 360).
If Koodo decided to change the retail price to $300 instead of the gift cards for activation, for example, then her Tab charge would only be $12.50/month instead of $15. Then Sarah could end up with a cheaper bill for the same phone, same plan, same time of year, and same Tab system! I would be totally cheesed to find that out.
They seem to be offering more "activation credits" and that sort of thing for activating with Tab than they used to, which does make more sense.
Obviously we don't have this problem for any phones costing a lot more than $360, for example any high end Galaxies or iPhones. Those are also high demand devices that cost a lot more, so sales are fewer on them.
Then again, we're used to subsidized phones here in Canada...