It doesn't step between anyone. It takes content that was delivered to a person's computer and post-processes it (or changes the settings of the machine). What I do on my machine with content I access should be up to me, and if content owners don't like it, they have the choice of negotiating a different deal with me before serving me the content. I view adblock as the equivalent of paying someone to paste paper over newspaper ads before you read it. (By the way, I don't use adblock or any similar stuff.)
More correct analogy: You are allowed to pay someone to paste paper over newspaper ads before you read it. You are just not allowed to hire such a guy inside the shop selling that newspaper, as per that shop's policies. Feel free to hire anyone outside the shop.
i.e. Google is fine with downloading AdBlock independently. They just won't allow it in Play store which is closed (unlike the OS, which is open).
The analogy does make sense, but I don't think that Google's stated policy applies under any reasonable interpretation. If they changed to a policy like "we don't allow apps we disagree with", then fine. But to ban these apps on the grounds they gave is disingenuous in my opinion.
Well, you could apply your own words here... if don't like it, you have the choice of negotiating a different deal with them before using the Play store.
