While looking for something else in my ancient FAR/AIM, I discovered that CFR 135.225 prohibits American part 135 aircraft from conducting an instrument approach to an airport unless "that airport has a weather reporting facility." That made me do a big double take, because Part 135 is commuter, on-demand, and mail delivery, the sort of thing that goes everywhere at any time in Canada.
I don't think that difference is a bad thing. As someone commented, the United States has a lot of instrument approaches, far more than Canada. Canada has more uncontrolled IFR flight. I cross-checked two pages of the index of a Canada Air Pilot (approach plate book) with the online aviation weather and found thirteen that have no reported weather. There's nothing prohibiting on demand flights from operating into those airports.
CFR 135.225 goes on to forbid an approach unless "the latest weather report issued by that weather reporting facility indicates that weather conditions are at or above the authorized IFR landing minima for that airport."
There is a condition that forbids Canadians to attempt an approach, but it only applies if the airport is equipped with an RVR (runway visual range) transmissiometer, a thingy that reports how far along the runway the pilots can expect to see in the touchdown zone. David has already blogged on the conditions under which the approach ban applies, so I'll just say if the touchdown zone RVR is below 1200 feet, and you have not declared in advance the intention to conduct a missed approach (for training purposes), you can't proceed to the FAF. Not a lot of airports have RVR.
Twelve hundred feet is a quarter mile, half of the half-mile advisory visibility that applies to most ILS approaches. I like that better than banning an approach as soon as the reported weather is at all below minima. Weather observations are always an approximation. If it's just below minima I'd like to have a look. Sometimes localized phenomena make a big difference.
I can remember approaching an airport that is served with no weather, but it is controlled and has an ATIS (does that count for CFR 135.225?) It was early morning, and the tower had just opened. The sky was clear, with calm winds and the runways were clearly visible. The ATIS indicated a visibility of an eighth mile in fog. I looked again and saw that the control tower was enshrouded in a little bubble of fog. It literally was the only fog on the airport. A VFR aircraft would be forbidden to enter the control zone. An IFR aircraft couldn't conduct a visual approach (because of the low reported ground visibility), but could request a contact approach, as it is governed only by flight visibility and a reasonable expectation of reaching the airport by visual reference to the ground.
Once on the ground, with that reported eighth mile visibility, you'd be forbidden to depart, because take-offs are governed by visibility. Fortunately it was a temporary phenomenon. Maybe they turned off the fog machines in the tower.