FranticFish describes my general feelings about gray-barred pages pretty well. Some additional thoughts....
The site in question is an industry body with 150,000 members.
I've observed in various industries that Google will often gray-bar the member listing pages, particularly if they're directory style pages. I don't remember having seen white-bars on these before.
Directories and pages that have a lot of links and not much other text tend to get gray-barred, I feel, because... if they are selling links, listings, legit memberships, etc... Google doesn't want a displayed TBPR to make them attractive to link or listing buyers.
Often, but not always, these pages only have generic text, like company names and cookie-cutter descriptions, which I'm assuming make them less valuable as linking pages.
Very often, business directory sites are form or search-driven, so the listings aren't SE friendly anyway. You might see if you can find some unique text on one of the listing pages, put it in quotes, and see if the page shows up in Google.