It's much easier to enforce an outright sanctuary, than try to enforce whether people are snagging or flossing fish (and other illegal methods). You're near the river with a fishing rod...pretty simple, you get a ticket. Period. There's no grey area, where a CO would have to examine every fish to see if it was legally hooked, and observe the unlawful act in person. It should be effective in shutting it down, just like you never see anyone fishing there between January and April under the current reg's.
OMNR's rationale for the closure is to protect the largest run of wild Chinook in Ontario. Although there are obviously social factors that were involved in making this decision, it's being done under the guise of conservation.