said by sbrook :

You cannot work legally anywhere except in the US and US protected territories, unless you obtain work authorization. The idea of "working legally" is without formalities.

There are many countries that will NOT issue work authorization to foreigners. Most countries will make you jump through hoops to get a work visa. One of the more difficult countries to get a work visa in for foreigners is the USA ... the limitations generally mean that unless you are absolutely gifted with PhDs up to your armpits, or you're a gifted sportsman, or an internationally recognized entertainer, your chance of working in the US in a timely manner for a prospective employer drop dramatically.

It's easier for a company to contract with an offshore support company than it is to bring those same workers to the USA.

said by sbrook :

Not quite, no. Whilst the US places no restrictions on Americans working in most places of the world, other countries may place restrictions on who may work in their country, and it is by no means a formality to get permission to work. For example, even though Canada is the US closest neighbour and trading partner, there are only a small fraction of Canadians to whom the US would grant a work visa.

It would be very presumptuous to assume that an American could work in any country because it's not up to the individual or to the USA.