That'a about it in a nutshell. They're not supposed to ship computers outside the USA, but some can be talked into it.
It has a lot more to do with the US Department of Commerce than Apple, but that's the short answer, anyway.
Refurbished computers must be serviced in the USA. Since the refurbishing process should be fairly hands-on and comprehensive, some people say that they are more reliable than new units. Still, DOA is not unheard of.
In short, it's a gamble.
If you're not familiar with shipping a variety of items from the US, there are caveats you should know about. Shipping costs will be borne by you (free shipping offers won't extend to Canada). You will need to add insurance, which ups the cost & GST.
Because it will almost certainly be shipped ground (UPS, FedEx, whatever) due to it's weight, you will have to pay a brokerage fee in additon to the shipping costs. Expect a minimum of $40 and it could easily be much higher. You may be asked to pay this before they will deliver it, or you may get a letter a few weeks later demanding payment.
You will pay GST or HST on the entire purchase, including shipping and any addtional fees. Technically, you're supposed to pay PST as well. If it isn't charged at the border, you're supposed to voluntarily cut them a cheque. It's not unheard of for a province to study import records and send you a bill 2 or more years later. Since you're from Alberta, this shouldn't be a problem. However, if it is imported to another province that province could (if they have an agreement to that effect with CCRA) charge PST which you (in theory) can demand a refund for. Rare, but it happens. Expect this to take months to sort out, if it happens to you.
Should you need service, you may end up paying GST and brokerage a second time when it's fixed; Canada Customs doesn't allow duty or tax-free status on repairs that could be performed in Canada. The GST (and a brokerage fee, again) will be applied to the shipping cost, so just because the repair is free doesn't mean it won't be an issue. Their idea of "could be" is whether anyone in Canada could have done the work, not whether it needs to be shipped out for warranties.
I would probably forget about it unless the difference in price is at least 30% (and I would think hard about it at that level; 40% is more like it). At 20% it's probably cheaper to buy new in Canada. Laptops would be less expensive to ship, but they do break more often due to the portable nature of the beast, and repairs are expensive so you will want to take advantage of the warranty.
It's a bit of a mystery as to why refurbished computers aren't available in Canada. My guess is they are, but the volume is so low that they get snapped up by dealers or other volume customers and never see the retail light-of-day. Another possibility is they go back to the US, and stay there; or perhaps parts are pulled and enter the service part distribution in Canada.