...or its a local thing, where anybody shooting from a bipod is considered 'F/TR' and anyone shooting from a rest is considered 'F/Open'.
I'm not saying its right... but I can see how it might happen.
Some people build their 'tacticool' 6XCs or .260 Rem to beat the .308 shooters in tac matches... and then don't like it when they get thrown in the deep end with the dedicated 'belly benchrest' rigs in F/Open. They're used to being at the 'top' of the food chain... not the bottom.
Around here, matches are generally small enough that very often, we don't even end up splitting out F/O and F/TR (not enough shooters of any one flavor for separate classes); everybody just shoots as 'F-class'.