Right, but if there are no other WiFi devices around, presumably the card would fall back into a much more passive mode that's likely to be in the intersection of all domain's requirements. ie. I doubt it'd come out with guns-a-blazin', pumping out 10W until it got a friendly tap on the shoulder that told it "Hey, you're only allowed 100mW peak around these parts."
Rather, if the supported regulatory domains had maximum transmit powers of, say, 120mW, 100mW and 80mW, in the absence of any guidance to the contrary, I'd imagine it'd stick to under-80mW until it got a whiff of what regulatory environment it was in.
After all, when you drive on a new local road you've never driven, do you floor it and go 65MPH until you see a speed limit sign telling you it's 30MPH? (100km/h and 50km/h if you prefer.) Somehow I doubt it. You probably look at the other cars and see what range of speeds they're going, and if there's no other traffic, you probably make an educated guess what an appropriate speed would be.