I asked the question,way back when,of a OHEP instructor who deferred to the OPP whose position was that due to the huge number of hunters taking to the field during the two-week shotgun Deer hunt,it was necessary to reduce the range of firearms purely as a safety factor. The "shotgun only" rule was enacted to facilitate that requirement for obvious reasons,especially,as land was being taken over for development and more and more houses are being built in rural areas. Local municipalities used the by-law process.,but,as such,was incorporated into the various FWCActs which have been enacted by the province over many years.That rule,in most southern Ontario WMU's,is still in force,but,has led municipalities to re-write their by-laws,relying on the provincial legislation as a blanket regulation. For the municipalities,it relieves them of the onus of firearms regulation,save for zoning.
While the shotgun Deer hunt is closed,as long as we have a Black Bear license,we can still use heavy rifles,even in areas where the FWCA clearly states that rifles of .270 or smaller may only be used for SMALL game. Hunting BIG game,such as Bears,isn't restricted and heavier rifles may be used.
It's wierd,but,that's the way the Act is written.