AFAIK there is no legal mandate that says a baseband cannot be flashable. The FCC will not grant certification to a device where the frequencies could be changed, but that just means it couldn't be sold directly in the US. You could easily get such a phone from overseas though.
I'm not sure how that regulatory structures works on other countries. I'm sure it's mostly similar.
I'm not sure how that regulatory structures works on other countries. I'm sure it's mostly similar.