How about this; you make a decoction out of carrot, then throw some lsd inside (class A substance). You then distribute it around, eventually something happens, police comes, and the prosecutor says, nah, it's carrot juice, just take the guy out?

Not quite; they will analyse it and then claim that you spiked it with lsd. It is the same with an aya decoction. You could had slipped the dmt inside, or it could have come from the plant, in any case there's a possibility to be charged.

Bottom line is that they can press charges if they want. It is a "grey" area of the law anyway. Officials can circumvent and bend laws to their own liking just as we (or a good lawyer) can do the same. And since it is a grey area people should be a bit more careful, basically act sanely (as opposed to getting out and shouting IT MAKES YOU TRIP AND IT's LEGAL, SO NO PROBLEM!!!:d )