Section 7.1 of the ANSI X3.60-1978/EMCA-55 Minimal BASIC Standard specifies that string variable names end in the "$" character. That
That particular string variable naming specification allowed the Standard to be backwards compatible with the 1968 implementation of Dartmouth BASIC (and with DEC BASIC, et.al.).
So it'ssupport for that type of string variable names is required. (whether or not any particular implementation could do otherwise, due to symbol tables or whatever...) There are commentshints on the web that specification compliance may even have been a government purchasing requirement.
And Shepardson Microsystems possibly didn't even start porting Atari Basic until after the Minimal Basic Standard was published in 1978.