LinkedIn just imported my whole gmail contact list without asking me

We all have seen the different posts on HN and on the web warning us about LinkedIn practices regarding contact importation, how they constantly try to trick you into giving them your whole contact list. But we think we know how to avoid them, we're better than this of course.

This morning I checked on the "people you might know" and 1 out of 4 people is one of my gmail contacts, with a nice "Invite contact" button.

They got me, I have absolutely now idea how they did this.

I've added some contacts yesterday using their (crappy / laggy) app, using the nice and round "+" button on people I knew, but never did I allow linkedin to import my contact book. (neither gmail to give it away btw ...)

Now I'm asking myself: how did they get me? I'm pretty savy, thus if they get me they'll trick others, less savy users. Without them giving their consent (or, as it should has append for me, unknowing they were giving their consent ...)

How can a company think this is a good idea to trick its users like this?