Hmm, I think you're onto something. Maybe they just don't get that the vulnerabilities are already there, waiting to be exploited -- they think that the person they've heard from actually broke something that will now make it possible for others to get in. I guess if you don't know what's going on, that's as reasonable a theory as any.
Another way of saying essentially the same thing: both parties believe that "extraordinary claims require extraordinary evidence." However, for us, the extraordinary claim is that software is secure; whereas, for them, the extraordinary claim is that the software for which they have paid so much money can somehow be insecure.
If I were really creeped out, I'd be even more likely to spend effort making sure the locks on my windows are secure, as now I know there's a menacing weirdo looking at them, so I'd want to be extra sure he couldn't get it.
Now, imagine that the menacing weirdo had included a return address on his letter. Would you report him to the police? If he got locked up, then that would be one way to make extra sure he couldn't get in.
And then the problem is solved, do you don't even need to fix the locks...
Imagine that you got a letter from a stranger, telling you that the locks on your upstairs windows didn't work properly. How confident are you that you'd take it as a helpful suggestion, rather than being completely creeped out by this menacing weirdo?
I think that negative responses to disclosure are generally grounded in a mixture of fear, mistrust, misunderstanding and arse-covering. Someone who discovers a vulnerability is seen as inherently untrustworthy, because why else would they be snooping about and trying out the locks? We think of computer systems as inherently insecure until proven otherwise, but they see their systems as fundamentally secure until someone comes along and breaks it. If you're fearful of technology, it's easy to hear "excuse me, I think your system is vulnerable" as "nice system you have here, it'd be a shame if someone broke into it". Denials and cover-ups are often the default corporate response, because being the bearer of bad news can be a career-limiting move in many organisations.