The issue is that you don't have an alternative option. If you're concerned with the possibility of a modified Face ID module, by all means go to Apple for those repairs. They do a great job with good customer service, and in many cases you can maintain your warranty.

However, many people would prefer to have another company repair a device that they own. I personally am okay with the risk associated with a 3rd party repair, because the prices are often significantly better. Hell, back in the iPhone 4 days I was able to replace my own screen with little to no difficulty.

There's a difference between "I don't want to have my phone repaired by a third party" and "I don't want anyone to be able to have their phone repaired by a third party." I respect the first statement, which seems to be where you're coming from. I'm not a big fan of the second one.