There used to be a proxy adblocker that did that, but I don't think it works anymore.
The Kindle browser uses a proxy to pre-render pages on the server in order to lighten the load of the device.
Could AaaS (Adblock as a service) be a viable business? I think I'd pay for it.
It probably could, however I'm sure it would be shut down quickly by advertisers and parties showing ads - after all, you're offering a service to remove their source of income. Unless you'd funnel part of your subscriber's income back to the advertisers, based on whose ads you block. But I'm sure it'd end up being too expensive for the consumer and too much of a hassle for the one offering the service.
Not to mention you'd force your users to funnel all of their internet traffic through your servers, and it wouldn't work with encrypted pages (which large parts of the internet is moving towards)
What legal leg would they have to stand on? There's no law that prevents me ripping out the ads of magazines that get delivered to me…
Tech laws are absurd.