Good question, especially considering that they did go on to say that if covid was found in this bat population, the cave would be closed off for human entry permanently. I didn't probe further but I suspect that policy might be based on what you said about evidence of human -> bat transmission.
Still, I do feel that closing it off seems like the prudent thing to do. It seems kinda myopic to have a policy to only close after the fact while knowing full well that Covid is zoonotic and mutations can in theory occur in a bat population on North American soil. At that point, last year's exercise of pointing fingers at Wuhan effectively becomes a historic unfunny joke because it's not like the US tests their wild bats every day and what can possibly go wrong w/ reactive policies... </shrug>
My overall point though is that even if we have perfect root cause analysis, people will still choose to ignore anything from informed recommendations to even appeals to emotion. As they say, it only takes one bad apple.
Also, I have to admit that message you heard is strange. I don’t believe there is evidence that humans can transfer Covid to bats, plus, if the sanctuary believes that, why are they letting anyone through at all? I would expect the consequential threat wouldn’t register as many people already believe the disease is not going away. I’m not saying wearing masks is unimportant but the cave warnings are a good example of how melodramatic speculation can be counterproductive when coming from officials. We won’t make progress by asking for perfect behavior to stave off doom.