Imagine that you're a company not necessarily known for security prowess. Now imagine that you want to be able to demonstrate to users / customers / investors that you take security seriously and work with reputable vendors. You could list off the vendors you use, but then they're just taking your word for it. Wouldn't it be better if the vendor listed you?
Alternately, perhaps there are common investors or board members exerting influence.
Or the even simpler answer, which is that they didn't ask.
We put logo privileges in our standard contract just to see what would happen, and we got it a bunch.
As a side note... I imagine well probably see some of those customers coming down soon.
But usually these lists of "customers" is just someone trawling the user database for @foocorp.com and anyone signed up from there makes the company a "customer".