You could make a case that an ISP or proxy operator that replaces advertising with other advertising is stealing from the operator of the original website. But blocking ads is the same as not looking at ads in the paper. If you're not going to act on them or read them you might as well block them for the bandwidth savings and speed improvement. Besides that, not all advertising has your best interest at heart, there is quite a bit of malware that uses distribution via advertising networks as their vector onto your machine. So now adblocking has the perfect fig-leaf: it will keep malware of your machine. This makes adblocking mandatory within organizations if they're savvy about this, why take the risk of having to re-image a bunch of machines or allow a bunch of drive-by malware to gain a foothold behind your precious corporate firewall if you can simply block it at the source.