You're right, that is strange that they don't ship to Canada. It sounds like in the USA and UK you can legally import prescription medicine as long as it is isn't a controlled substance.
From united pharmacies website - http://www.unitedpharmacies.com/faq/
Why don't you need a prescription to sell your products, even though the same products require one over the counter?
We are an Oceania-based company and operate under different laws and regulations. Any non-controlled drugs can be legally imported into the US and many other countries by individuals for their personal use (depending on local legislation).
Is it legal to order medications like this?
It is legal in most countries including the USA and the UK. If you have concerns please check with your local authorities. We will always let you know if we think there may be a problem with customs in your particular country.
Maybe Canada has a law against importing prescription medications even if it is a non-controlled substance. NaltrexoneRx.com might ship to Canada but they are ridiculously expensive - $86 compared to $16.80 from alldaychemist or $18.50 from United.