Actually it would be quite straightforward to reverse engineer a math model if the full or partial test results were public. Think about it - you'd have the game's pay-tables and RTP published by right and then you could pretty accurately work out the parse sheets and reel mapping from the pay frequencies listed above. It looks to us maybe like the partial pieces of a puzzle, but to the maths people in a development team pretty easy pickings.
You even have examples like 2Winpower reverse-engineering numerous Novo and Playtechs based just on pulling a few million results from the RNG and unravelling the code to produce some pretty accurate copies. Obviously that's a criminal enterprise and legitimate developers will have a code of conduct and trademarks/patents to protect them so wouldn't attempt it anyway. But yes, it's perfectly feasible.
Even on here you've got enough info just gleaned from RNG-tapping to reproduce the MG IR/TS2 games quite easily! (Search kktmd posts)
Think of that pay spread info from JFTW as a 'slot Sudoku puzzle'.
Yes, I agree, it is easy for them to copy most games. Even easyer to get the frequency of a 1000x win. It may save them like 3 min of work to see it posted.
But most players need it and cant get it.