I'm sure if the boss gets to find out the employees gambling habits and realise that he is addicted to gambling, even if the boss used to trust is boy their will be this strange feeling the boss will have concerning his money , if the money is in safer hand.
My Boss would not trust anyone who would gamble or drink or do drugs, not at all.
For him - the clear mind gives clear direction - and an addicted gambler would be crazy for money. So YES! It is a cause of concern for our office if a person is gamblor he would not be in our office.
This is the case of boss not taking to such habit. Like some who have that kind of mind set that if you take alcohol then your eyes will be clearer and alertness will be high, then such person won't see anything wrong for the employees to do that when he becomes an employer. So your boss is acting very proper in such way by not allowing it. A Gamble who has the custody of money will go use it to gamble one day when he can't find a personal money for it.