Actually, I also buy them as toys.
As for MP Convoy, in Japan MPs aren't marketed at children - they are marketed as "Ages 15 and Up." I think that MP Convoy's smoke stacks are okay under Australian toy safety laws though because we've had two versions of that mould sold locally here (MP Magnus and MP Convoy w/ trailer - both Target exclusives). Afaik Hasbro Australia didn't market them as "Ages 15 and Up" but I'm not entirely sure as I don't have either of those figures.
Either way, the smokestack issue isn't one that falls under replica weapon - it's a matter of child safety.