The thing being stolen is the advertisers ability to advertise, which in turn pays for the platform. So, it is stealing from the platform.
Also, if you take a quick look at the pamphlet and throw it away, that's the same thing as looking at an ad and ignoring it afterwards. You were still looking at it, so the ad did its job.
Btw, don't get me wrong, I also use ad blockers for a lot of things. But I do pay for anything that I use for a good amount of time, like Youtube, video games, movies or music.
