I was trying to explain your point, not disagree with you.
You did, however, misinterpret the analogy. There is nothing wrong with buying at full price and reselling, but the cap is on to prevent a rep from company B buying 100's of items from company A at the promotional price of £3.99 to simply put them on his own shelf at £9.99 when company A's offer ends.
It is NOT how the promotion was meant to be used/abused so company A does its best to prevent anyone other than a 'genuine' customer (i.e. someone purchasing for personal use) buying all the stock and thereby denying access to those for whom it was intended.
There also has to be a line between arbing and trading, although, to my knowledge at least, nobody has tried to define it.