Not saying you're wrong, but I'm not fully onboard with this either. I purchased 6 corals in a live sale, after I received them I had one that developed RTN and was gone the morning after receiving them. The rest of them were doing good. The vendor was awesome and issued me a credit and I thought that was great since there was no way of knowing exactly what the cause of the RTN was.
I also discovered that another person purchased that same type of coral in the same sale along with 17 other corals. His also had RTN and was gone just as quick as mine. Again, the only one he lost. This tells me that the cause was from the vendor.
The cause will not always be from the vendor, but sometimes it will. Coming from a vendors view, yeah it's not fun to take a loss when there's a good chance that the client was at fault. Coming from a client view, why would I shop with someone who possibly sent me an unhealthy coral. So there's pros and cons no matter how you handle it but, everywhere I've shopped, vendors have given a more than reasonable guarantees.