"Nobody is doing so. Clearly the results of mutations are subject to natural selection. The problem arises from the assertion that those mutations arise from some cause."

Overwhelmingly, the results of mutations are that the mutations will not survive, way before the point where natural selection gets hold of them. A mutation may be a nonsense or lethal combination for a start. Then again, even if a mutation codes for a potentially-viable trait, it may well be recessive and not expressed. A tiny minority of mutations will ever find phenotypic expression, and only then will natural selection, er, "get to work."

The assertion that mutations have causes is demonstrable in that certain environmental conditions or chemical agents can be shown experimentally to cause mutations. Blimey, even fag smoke is known to contain mutagens. We all know that unprotected sunbathing (exposure to UV) can cause mutations in skin cells. Asbestos fibres can trigger cancerous mutations in lungs and surrounding tissues. And so on. The fact that frequency of mutations can be increased by subjecting tissues to these agents surely makes us suspect that mutations in general have causes. I don't know why you don't see this point. I'm not saying I'm right but at least I'm not being counter-intuitive.