<human>: Can you condense the text into a few key points?

Our cat (large male, 8 kilos, 5 years old) recently went through treatment (Catheter and 4 night stay at the vets) for struvite crystals caused by FLUTD. 

Our vet has told us that we should now feed him Royal Canin Urinary SO canned food exclusively to prevent further issues and to gradually get his weight down a bit. Now, we're happy to do so if this will keep him healthy, but a few things we are a bit unsure of:

- We're skeptical on this being a so-called "prescription" food. That's because when we asked if our other cat could eat the same thing to make things easier at feeding time, they said it was fine. Why is this food considered "behind the counter only" if it's fine for normal cats to eat too? I don't believe the vet is trying to scam us by buying their (expensive) food, but I'm skeptical about the pet food industry in general and I'm not sure if vets aren't influenced by the sales pitches by pet food companies.

- We're unsure about the cause of these crystals. We've gotten conflicting answers from the vet each time we've called; we've heard that it's dehydration, minerals like magnesium, as well as the pH balance.

- The food we were feeding him before was grainless. I was under the impression that we should be feeding our pets foods that have the least fillers. The prescription Royal Canin food, while expensive, seem to be chock full of the fillers that the everyday grocery store brands do. As said, we'll be glad to feed them this if it'll prevent crystals forming, but will we be faced with further health issues because of the crap that's in these?

- Has anyone used any alternatives to these prescription foods with success? We've also seen supplements for urinary tract health to keep pH down as well.
<bot>: Our cat is prone to struvite crystals from FLUTD, are there alternatives to the prescription foods that vets suggest?