Many, many, many fighters use diuretics to make weight.

Roberto Duran used to use then frequently in his day.

Ricardo Mayorga tested positive for them before as well.

IMO its not a huge deal.

Yes, it is unfair, but it is not as bad as loading your gloves or using steroids.

To be honest I think a lot more boxers use performance enhancing drugs or have, and just have not been caught due to taking masking agents and cycling on and off.