I hold a teacher's certificate from Europe, where I grew up and taught many years. My certification is not accepted here just like that. I have taken lots of extra classes, but still I am nowhere close to getting to teach here in US. I had to sign a promise to become a citizen in three years also.
So there are regulations in every country, why do you think Americans should be able to go and teach children in other countries just like that? As far as I have experienced, USA is the toughest place to come and work as a foreigner.