Gooki , They state on their websites that the offer is limited to one person, ip address , household. (But I like your thinking)
It would breach the bookmakers T&C's to do use those websites and they are quite particular about how you fund your account. However we recently moved and I have been going through the offers with my girlfriend and instructing her on how to do them,then splitting the profits and I may do the same with my mum . (they both have no interest in gambling and the gf barely pays attention as I am advising her, before someone accuses me of creating a gambling addiction)
Grantmeaname - A lot of people don't even use the offers to lock in profits by backing and laying the same outcome. They simply bet on an outcome and can win or loose. They also then go on to gamble with their own money. I also agree that a lot of people will get hooked in by these offers, again as mustachians I hope we understand the maths behind gambling and that it is a pointless exercise.
Matched betting is not gambling, there is no thrill in the win, I don't even watch the games I've placed the bet on as the result does not matter. You simply log in to either the exhange or the bookmaker after the game finishes and withdraw the profits and move on to the next bookmaker. (you also know the amount you have won prior to the game starting)
Arbelspy - I like the link you posted about the Vegas offer, If you were traveling that way it might be fun to give it a go.
Eric - I agree, it seems strange that in a country such as yours, with the freedoms you enjoy that you are not allowed to gamble online. Is there reasoning behind this?