Kap'n, that's because most people are taught Treble Clef in grade school and can still remember it. While it may not be puritanically correct to write that way, it accomplishes the job. Technically there's nothing wrong with it, but it can make things a bit confusing.
The 22 fret guitar is just shy of a 4 octave instrument (not counting string bends). The range extends from the E below G on the Bass Clef to the third D above Middle C on the Treble Clef. So it fully spans both Clef's and extends beyond each.
Code:
E__________
C__________
A__________
F__________T
D__________R C
B__________E L
G__________B E
E__________L F
C__________
A__________B
F__________A C
D__________S L
B__________S E
G__________ F
E__________
____________________________________________
Code:
E__________
D 1 string 22 fret
C__________
B
A__________
G
F__________
E 1 string 12 fret
D__________
C
B__________
A
G__________
F
E__________ 1 string open
D
C__________ (middle C)
B 2 string open
A__________
G 3 string open
F__________
E
D__________4 string open
C
B__________
A 5 string open
G__________
F
E__________6 string open