Re: why have ANY upper limit?
Alphanumeric has 36 possible values per symbol, so just more than 5 bits. Alpha only, which I prefer for typing, has a bit less (26 values in plain ASCII, not counting case). Presumably, padding the length with random characters to be somewhat more than the number of bits in the hash of the password, will mean that your position in the hashed space is also pretty well random. And so adding more characters is not useful... unless and until they switch to a longer, larger space of hashed values.