Richtext : why an US-ASCII formatting command ?

I really don't see why an 'US-ASCII' formatting command is defined.

Given that :
1- richtext starts in us-ascii ;
2- any formatting directive, including of course those switching character
   sets, must be negated ;

it follows that 'us-ascii' can only be used to switch from us-ascii to
us-ascii !!!!
                                                              /AF