In the US, they don't have such stringent fuel standards as we do in Europe.
The fuels have less detergents etc in them (which is SOME of the reason why it is cheaper).
As fuels in Europe are generally better, it means that products like seafoam have much less of an impact on cars that have been run on European fuels.
Also didn't seafoam have a big court case against them for making claims that they couldn't back up?
Having said all this

I have in the past used redex, and letting the engine suck it in through a vacuum hose. It creates lots of smoke
Actually you can do the same thing with water which also creates lots of smoke!
Thought it may have made the cars work better (think it was mostly my old m10 and volvos...) but doubt it made any more different than a bit of italian tuning (ragging it) on the motorway.