The countries listed are the ones of the Schengen Area - and the CDC had already issued a travel warning against those exact countries, there as well excluding not only the UK and Ireland, but also Eastern European countries that aren't in Schengen, such as Romania, Croatia, etc. Which sort of makes sense considering that outside Schengen you have at least some kind of border controls, inside not so much. And the exception for American citizens is presumably because it's much more dicey legally to block American citizens from returning.

If this was one measure among many in a larger comprehensive plan to deal with the virus, following expert advice, I wouldn't really have a big problem with it - even if Trump's motivations for supporting it may be suspect and his speech announcing it was a trainwreck as usual. But certainly, if he thinks that this is going to make the big difference in combatting the coronavirus in the US, then that confirms what we're seeing increasingly often these past years: the president of the US is an egomaniac imbecile and what's more concerning, the people around him can't guide him towards something resembling competent administration anymore. Not that they were ever all that great at it, but it wasn't this bad in the beginning.