Mass market subscription services won't offer ad-free precisely because of the mass market exposure available to advertisers. You need to look to niche services instead, where a critical proportion of subscribers (say 30%) won't tolerate ads as opposed to 1%. Maintaining an ad-free option in such a case is basic business sense. Not only that, you'll find the ads-on tiers are more respectful of people's attention and intellect when it comes to ad content and presentation.