That list is very helpful and this problem has come up many, many times with most of the agencies, not just CanStock.
The lack of an opt out is a big issue and the lack of transparency - with Veer, for example, you don't know which sales are partner and which not. I think 123rf is the same. As some of Veer's partners sell the images at much higher prices than Veer does, but we get only the standard royalty, I think the reason to make the list secret is the agencies try to avoid problems with the artists over their various cash grab deals. Do a search for Veer and Alamy and you'll find several threads here on that ill-fated (now removed) deal.
CanStock does show distribution versus regular and instant in its stats, which is a small step up from some agencies. Getty/iStock does have some partner/Getty360/main site separation of stats, but in general they're awful about providing sales details. Even SS won't provide us with sales details for SOD sales. Before BigStock was bought by SS, they did some deals that people found out about - not via them - and the prices were very much higher (which obviously meant they didn't want it known you could buy the same stuff for less at BigStock). When contributors squawked, they said that we made extra money too so it was all good. We still squawked and they gave us an opt out. But by the time SS owned BigStock and imposed the insane royalty schedule for the subs program, they refused to provide an opt out.
And of course there was the Google/Getty deal about a year ago - lots to read about that, here, on Sean's blog, on my blog, on all sorts of places.
It shouldn't be this way but as they agencies became bigger and they figured they could get away with it, they work for their own interests and bottom line first and foremost.