My guess is that the part of the sign that indicates when the 30 speed limit applies, has gone missing. Or someone screwed up. Or someone is playing games.
min max
min max
That isn't the way minimum and maximum speeds are supposed to be signed according to the international convention (which Canada and USA aren't part of, which is why our speed limit signs don't look like that).
UK vs rest of Europe has same issue. Ireland vs Northern Ireland has same issue and no border checkpoints! You are expected to know what country you are in and what unit system they use. When you cross a border there is a big sign board explaining (in symbols) all the default restrictions that apply in the absence of explicit signs. You are expected to know!
You have more faith than me about the ability of many Americans to understand that anything outside their own little bubble exists.
If you think I was joking about Americans showing up the Canadian border in the summer with snow gear....sadly, I was not. I used to cross the border twice a day every day for years. It was a thing. Seriously.