I agree these should be a catch and release format but unfortunately, people can and will cheat.
I've heard of taking a bump board and making it smaller with a good photo app.
Personally, I think it should be CnR, if you choose and have a set size i.e. 15" - ??" and everyone that submits one in that size, go into a draw for 1st - 3rd place $$.
I think it's also a missed opportunity for some groups holding these, to have a measuring tape sticker made up, with sponsors labelled. I realize this would making it easier to cheat but with the suggestion above, why bother?!? I know I'd keep it on my sled for future measuring.
Like you, I would like to buy and support (charities / fundraisers) and have reluctantly done so, only to possibly win a draw prize as I'd never weigh-in a fish. Actually, I seem to not be able to catch any so that's an easy decision.
Should I have supported it? Not really! But in all honesty, it was what got my buddies and I out and probably would not have otherwise that day.
The potential fish kill is too much and hopefully the thought of "prizes for biggest fish" for these will stop.