Domains are intellectual property? That's a new one! There are a small number of businesses who can't register and collect because of the services they provide (there's a
list on the CRA website -- I don't see anything resembling a domain name there, or anything resembling intellectual property either for that matter).
You can look at a domain as a membership, a subscription, a license, all sorts of different things, but they're all taxable. You pay GST when you register or transfer a .ca domain name, and even in the TBR auction process.
The bottom line for me is that the client pays the tax (and if they're a business they don't care because they just get it back and pass it through to their client), and you save money by collecting it, so it just makes sense to do it.
I have a family member who was running a successful business with a number of employees, who thought their services were not taxable, but the CRA disagreed. They owed the feds an amount in the six figures and ended up losing their house and most of the business over it.