The reason they do this is that before they implemented this rule some players would bet equal amounts on red and black in roulette, or the pass and don't pass in craps. This would allow them to earn a bonus without putting much money at risk. So the online casinos added this rule in an effort to keep these bonus players out. Casino on Net does allow play on any game but reserves the right to refuse the bonus if the player bets on opposites, which I think is a more reasonable solution.
You are absolutely right that a casino should honor a bonus even if the player bets exactly the player requirement and not $1 more. Unfortunately many Internet casinos owners don't think they have to abide by their own rules and make them and break them as they go. Two years ago the Golden Palace rounded up all the players they considered to be not be giving enough free play and locked them out of their accounts. I know this because I had one of the accounts. After a lengthy audit they reopened some accounts but seized the funds of others and donated them to charity. This was despite the fact that the players followed every rule. It just goes to show that there is no credible regulation of Internet gambling and that appearances are very important. That is why I advise players to not even look like they are only after a bonus and to provide plenty of extra play.