This would place almost as much burden on the golf courses that offer any kind of discount as it would on the raters themselves. The courses would have to provide the determination of how much "income" the rater is receiving from their discount and most likely track and have forms available for verification--I guess the good news for the anti-rater crowd is that a lot more courses would stop offering the benefit! The courses are the only ones that have any idea of how much "benefit" the rater receives from a free round.
From what I know of the tax system, if these "benefits" were to be considered income then the expenses would be miscellaneous itemized deductions since rating golf courses would be seen as a hobby and not a business for most of us. There's a floor on those deductions (2% of AGI I think), so not all of the expenses would be deducted. For anyone that doesn't already itemize, they probably would pay some tax, although the amount would probably be fairly minimal. It would be an inordinate amount of work for a fairly small amount of additional tax dollars. And of courses this wouldn't just apply to raters, but golf professionals, architects, superintendents, and anyone else who ever gets comped/discounted for rounds of golf for any reason.