Of course, we want to sell...as long as no one knows about it

I just had an email from a client of ours who are, by their own (unsolicited) admission  "big fans of your books!"  I was flattered and said so. Then I asked them if they would consider giving me a testimonial that I might use in advertising. 

"uh....no, we think we'll remain incognito...."  they responded.

What is that about?  The material is worthwhile in teaching them to sell effectively, but they don't want to appear as though they're studied about it?

It's a thing I've noticed about some salespeople:  they believe that their customers believe that if they're 'selling', it's tantamount to taking advantage of them.....