@BigT Why don't they want single people placing multiple bets? Surely a bet's a bet... so they've got just as much chance of losing as winning, no?
Well I'd say paranoia. They would say that there is a disproportionately higher chance that such people are engaging in activity that guarantees profit and that has the potential to do the bookmaker out of profits because you won't be exhibiting the same behaviours or thought processes when placing your bets when compared to the people (mugs) they make more money out of. So yes a bet is a bet but they stand to make more profit off people not like you so they'd like you to go away.
Consider the qualifying bet. That tends to be as low odds a possible so that your qualifying loss is small. Lower odds means more chance of you winning from the bookmaker. Now compare it to the mug who just puts a bet on his/her favourite team/horse/first scorer etc. Often longer odds meaning more chance of the punter losing and the bookmaker winning.
Most matched bettors put real money on short odds qualifying bets that are quite likely to have the back side winning and thus the bookmaker losing. Then your free bet goes on something much longer to maximise profits. Invariably this goes to the exchange because the lay tends to win but the bookmaker doesn't win anything because it was a free bet, not your money they keep. so matched bettors tend to be a drain.