Can you find any cases in Canadian history of it happening? If it hasn't happened before, most likely it won't happen going forward. It's easiest to just go after the contractor so that's what the CRA does.
It's the same thing. It's not the purchaser's duty or job to know what tax they need to pay when buying a product or goods and services. They get quoted an all-in price, which they pay to the contractor and then the contractor makes the HST payment. If the contractor is not paying proper HST to the government, that is on the contractor.
If the purchaser was the one making HST payment directly, that would be a whole other story.
In real life cases where contractors have been convicted of tax fraud by not reporting proper income/HST, has the CRA ever come back to the purchaser to recover the missing HST amount?