Airlines better brace themselves for emboldened passengers doing what they like and when challenged by the crew demanding recompense
Pax cannot do what they like. If its illegal the judge will determine their sentence or fine, as many of the bogan brigade have already found.
Mr. Dao was quite within his legal rights and did absolutely nothing wrong to receive what he ended up getting aboard the aircraft. Once he said he wasn't volunteering that should have been the end of the matter, and the airline should have found another individual who would have been happy to take whatever was on offer to give up his/her seat.