It is my understanding that those guns, as well as other Browning FN guns were never imported here prior to WWII for the reasons you mention. Starting around 1954, the Model 1910, the Baby, and the Hi-Power were imported, but the old .25 never was. The Model 1910 and the Baby were banned from import in 1968, but a "target" version of the former was on the market for a while after that.
But thousands of American soldiers have been stationed in Belgium and at one time could buy handguns there and have them shipped home. In addition, the Germans used thousands of FN guns and these were taken as war trophies. Some surplus military guns, like the Model 1903 Swedish contract guns and the Dutch Model 1922, were also imported.
So the little Browning is rather uncommon, but there are a goodly number around; they are excellent defense pistols within the limitations of the caliber.
Jim