Smoking weed doesn't exactly leave a paper trail ....
unless your NETeller's are to Carlos Escobar with a memo of 'ganja' there's no way to catch you other than to see you doing it. (which they will arrest you for).
It's much easier to go after gamblers because we're moving thousands of dollars back and forth through the same limited number of channels (NETeller, Firepay, WU, etc ..). Not exactly the same as buying a dime bag from Joey 'Crack Head' in the alley.