Companies legally prohibit 3rd party repair as it affects their IP. There are a lot of companies that void all warranties if you try a 3rd party / DIY repair. I've had first-hand experiences with Apple and Samsung in India. There are lots of other companies, see - https://repair.org/industries/main/
Coming back to your situation, a magnetron might not be patented but that could just be one of the many components that renders a microwave non-functional and a lot of those other components might be patented. A microwave repair shop cannot possibly be profitable if it only fixes magnetron issues. It would want to fix most, if not all, issues with customers' microwaves. But it can't as companies legally prohibit 3rd party repairs. Sure, people still reverse engineer and fix things by themselves but these laws are major blockers for 3rd party repair markets to flourish. Despite the laws, there are pockets where this still occurs (e.g. China). And should the ban on 3rd party repair be lifted, it's going to be a different ball game and opportunities could bring us a lot more big businesses like iFixit.
It seems totally reasonable for 3rd party repair to void warranties. 3rd party repair will be of varying quality and it doesn’t seem logical to make the vendor liable.
It's not that simple. The point is vendors empowering 3rd party repair shops with knowledge and tools than monopolizing repair market which leads to higher repair costs, lower product lifespans, forced product upgrades, etc. - basically everything right to repair associations are fighting for.
Coming back to your situation, a magnetron might not be patented but that could just be one of the many components that renders a microwave non-functional and a lot of those other components might be patented. A microwave repair shop cannot possibly be profitable if it only fixes magnetron issues. It would want to fix most, if not all, issues with customers' microwaves. But it can't as companies legally prohibit 3rd party repairs. Sure, people still reverse engineer and fix things by themselves but these laws are major blockers for 3rd party repair markets to flourish. Despite the laws, there are pockets where this still occurs (e.g. China). And should the ban on 3rd party repair be lifted, it's going to be a different ball game and opportunities could bring us a lot more big businesses like iFixit.