"In the case of strings, however, it should be pretty obvious why: a string represents a sequence of characters, and does not associate a value to any character, so there is nothing to compare."
Actually, the TI-89/92+ calculators do extend the ≤ and other logical operators to strings (using their dictionary order). Of course, those calculators are much more open about the internal structure of their commands (e.g. you can use char(n) and ord(s) to convert numbers to characters and back); sorting by tokens might not be too intuitive on a TI-83+ (why is "Horizontal">"Vertical" again, you'd ask?)