This is a false dichotomy and an unfair one. Firstly, fastmail is a paid product that may show you graphic ads if you are using their free tier. (https://www.fastmail.fm/help/advertising_web_interface_ads.h...)

To those who are arguing about advertisements not showing up in the Fastmail's paid tiers should also realize that there is the Google Apps for Business accounts, where you don't see ads. It also beats the popular 'if you are paying you are the product meme'.