I think Jim-J probably put it best:
"4 stripes? He's acting. He's in character...."
Travolta has an arrangement with Qantas and the 70's era uniform and aircraft paint job are part of the image they have agreed to promote.
I suspect that he consider's the Captain's rank as being an appropriate symbol of an airline pilot and that he wears it not as a snub to the professionals but possibly as a tribute to them.
Yes he's taking a mild liberty, a few too many centimeters braid, but as mentioned by others, its his show and its not a bad one and that gives him the right to play by his own rules.
I wonder how much smoke that thing puts out on take-off?