1v1 etc. as long as you're making attempts at the objective and/or killing the other party/parties is perfectly legal. planting for xp, letting a friend defuse for xp, is manipulation, and is not allowed.
if you plant and decide to message instead of preventing your opponent from defusing, that's a conscious choice to let him defuse in my book, and is manipulation. if you plant, walk away to make a sandwich...it's understood you're letting him do it. the fact that you were doing something else doesn't make you innocent.