If they can claim their losses and expenses it should be taxed,...

If they can claim their losses and expenses it should be taxed, if they are only taxed on their winnings and losses are not deductible from future income then it should not be.

If you gamble over say 500k/year then you should have to declare at the beginning of the year whether you wish winnings/losses to be assessable, if you don't declare then it should automatically be assessable.
(This excludes casino's as obviously they are negative expected value so no professional organisation would use them in attempt to make money and they are only for entertainment value, ie. winnings/losses should not be assessed.)