Old Astro version has been reported to be illegal due to the frequencies used between the dog transmitter and receiver in UK New Zealand France Italy Austria Canada Australia Namibia
Probably they're legal in Germany Belgium Nederlands Brazil Finland Sweden US ( but it depends on states: it's illegal in California) China Taiwan Russia.
In Some countries You might be able to buy them legally but you cannot use them legally.
The frequency of these devices is supposed to interfere with the emergency services radio bands.