Did you ever get an e-mail from a casino saying, "You recently played with us and lost $200. Upon review, we determined that your play was not in accordance with our Terms & Conditions, so we are refunding your deposit"?
Of course not.
Yet, if you win you are very liable to be told that you violated the T&Cs, your winnings have been nullified, and your deposit is being returned. (That, at least, is the norm in the industry. A few casinos go beyond that and actually keep your deposit as well.)
What this boils down to is a policy of "You can lose, but you can't win". Casinos have been known to withhold winnings because they feel you bet in a manner which violated the rules of a bonus, that they do not believe the proof of identity that you sent them, that they believe you coordinated your playing with others in a group, or even because they don't like your method of betting (your bets are too high/your bets are too low).
Can you imagine this happening in Las Vegas or Monte Carlo?
Yes, land casinos also have rules. There may, for example, be table limits. However, if the casino accepts your bet, and you win, you will be paid -- even if the bet was above the limit.
Many casinos do not allow card counters to play blackjack. If they spot you as a counter, you will be asked to leave -- but the money that you have already won will be given to you.
Why should on line casinos be any different? Indeed, the only reason they can get away with this is that they are often in remote jurisdictions, far from where most of their players live, and it is almost an impossibility to sue them.
The casinos say they have to protect themselves. Fair enough -- but it should be done in an ethical manner.
To start with, they should establish your identity to their satisfaction before you are allowed to play. You should not have to wait until you try to withdraw before finding out whether they believe your documentation or not.
Secondly, they should make it impossible for you to place a bet which is against their T&Cs. A few instructions written into their software would take care of this. If you are playing on a bonus which only allows you to go to the slots, the software should make it impossible for you to play any other game until you have met the bonus' requirements. Likewise, if the bonus says that you are only allowed to bet a particular percentage of it on any one roll of the dice, the software can be instructed to refuse a higher bet.
We come to on line casinos to enjoy ourselves -- and, with some luck, win some money. We are willing to take our chances of losing, but we should never be put into a position where we can lose but not win.