Most common reason for denial of entry into Canada is DUI.
No expert, but Canada denies entry to everybody convicted of an offense equivalent to one of their felonies. For them, all DUI convictions are felonies. Meanwhile, with no draft, a convicted draft dodger would be let right in because draft dodging a non-existent draft isn't a crime there.
There is a way to petition to be allowed in, submitting that the crime is old, you've served your sentence and you've reformed. It pretty much requires a lawyer familiar with the process and costs several hundred dollars for a DUI.
Point of all this? Manning should have realized that by reading the rules at all, and the chances of a waiver are hell no.
Canada's immigration officers did their job by the book.