Would you like signs with those chars?

That isn't the problem, as far as I can tell. The problem is that, if you accidentally have a character with the high bit set (because you're using ISO-8859-1 or Windows-1252 or some other 8-bit ASCII superset instead of* UTF-8), then it will sign extend and you will get nonsense. Since those non-Unicode encodings used to be fairly popular, compilers started warning on this, even though I believe there's nothing in the standard that explicitly requires a diagnostic on conversion from char to int. Taking care to do this is good practice, because if you offer a 7 bit channel, people will (ab)use it as an 8 bit channel whether it is intended to support that or not.

If you can find an environment where someone actually declared isalpha and relatives with arrays rather than ints, I would be very surprised, because the standard specifies that the argument must be an int, and any subsequent int-to-array conversion is the callee's problem.

* This problem is theoretically also possible if you are using UTF-8, but in that case, you'll get nonsense anyway, because UTF-8 has to be decoded before you can call functions like isalpha on it - and at that point, you've already widened everything to 32 bit, so hopefully you did it correctly.