It kind of is, though, isn't it? The online ad market is theoretically super efficient - if the market clearing price for an ad is $1, and then you add a 10% sales tax, it no longer makes sense to bid / pay $1 for that ad, knowing that you will be out of pocket $1.10. Instead you would bid about 90-91 cents, knowing that it would get marked up to $1 with tax. Net effect is that Google only makes 90 cents on that ad instead of a dollar, even though they aren't directly paying the tax.