Almost all of Syncs business model is ads. The free version has ads and almost everyone who pays for it is doing so to remove ads. Which is just rewarding the implementation of ads. I also disagree with the concept of profiting from free user content with ads like Reddit does. Which was Reddits primary goal of preventing third party apps. They wanted the ad revenue themselves instead of third parties getting the ad revenue.
The only way this can be acceptable is to not have a free ad version and only have a paid version. That way you are paying for software and not paying to remove ads or profiting from free user content.