It also doesn't say "Once per turn, after charging, [...]", as many feats, powers, etc. often do.
Ninja'd!
Damn, was going to say exactly that.
"After charging, you can shift 1 square before your turn ends."
The "before end of turn" stipulation is how long you have before the ability to shift is withdrawn, it doesnt actually imply once per turn.
1. He charges. Tick
2. He shifts. Tick
3. spend action point. Tick
4. He charges. Tick
5. He shifts. Can He? Read the description. Description sais "after charge, before end of turn".
To be honest, my gut feel sais that it is only meant to be once per turn. The point about it NOT saying each time you charge is well received and probably not present because when it was being penned, they were not thinking about the fact that someone can charge twice per round by spending an AP...they were probably assuming it was possible to charge once per turn, so the once per turn stipulation is not required.
Wouldnt be the first time wording hasnt been 100%, otherwise there wouldnt be so much damned errata.
However, by RAW the current wording (and therefore the rules) does NOT restrict it to once per turn.
So, my interpretation
By RAW : Twice per Turn
By RAI : Once per turn
If this was my game, I would probably house rule once per turn and make it very clear to the player, but Im not your DM
Either way, this is an interesting point. Im very interested to hear other input to it.