You are right that there is no way to know if a gore movie was build for sale. That's why law enforcement is supposed to deal with each case considering the unique circumstances. logical consistency is not the only thing that laws consider(in fact its one of the least important aspect). What is immoral is not necessarily illegal and vice versa.
The reality of the situation is
1. Gore porn watchers have plethora of options to enjoy stuff that is just randomly filmed or even filmed with intent but violence was not commited by producer. This snubs the chance for an industry and it is a fact that deliberate gore video production is much smaller than child porn.
2. Gore porn consumption is fundamentally different from child porn consumption. Just because both are immoral doesn't mean they can be compared. Different psychological phemomena. One is linked with anti social behaviour related to many kinds of psychological disorders. Other is a fundamental sexual urge. This changes the implications for the subject who is consuming either. The state has to use different approaches to decide how to deal with such individuals and which ones to just leave be. Based on the implications of their habits for society.
3. Child molestation is different from violence as well. The risk-reward equation is unbalanced. This is reflected in the fact that a child porn consumer may actually molest children he comes in contact with, but it is highly unlikely even for a grotesque violence consumer to just go on a killing spree. The psychological relation between watching it and doing it are different in both. And in case of child porn it is very closely related.
4. There are healthy ways to consume violence. Everyone does so. We have action movies, movies like Saw etc. And so there are healthy ways for pedophiles to deal with their urges. If they are moving to illegal activities to fulfill those urges the problem lies with them and leaving them be to do so is unwarranted.
5. Most importantly, why is it illegal? Simple, a child porn consumer, even if he indulges in freeware is aiding and abetting the exploitation of children. Nobody just uploads the child rape film he bought on the dark web just because. He knows there are consumers in the network. And those consumers can and do act as distributors in their turn. They are perpetuating a system of exploitation even if they are at the recieving end of it. Plus laws against possession help law enforcement nab distributors. You only downloaded and are not a distributor? Too bad buddy, you should never have downloaded to begin with. Its illegal for good reasons. Try to deal with your pedophilia by legal methods instead of watching literal child rape.
6. By above logic you'd say ban snuff films and I agree. But you say that it can't be differentiated from regular gore porn to which I disagree. And that's what investigation is for. The thing is, logistics in both cases are different as well. Nab a cp watcher and you have 100%hit the nail. Nab a gore watcher who may possess snuff and most likely you are wasting your time.
There will always be snuff, cp, murder, rape, theft etc. Law enforcement has to use their limited resources judiciously to maintain order and curb crime.
People who here believe that law is made to follow some arbitrary criteria of logical consistency clearly have little idea about how laws, culture or society work.(this is for you as well @Mainländer in case you needed a revision.)
Ps.. I am not a glow nigger. I am not even burgercel.