Our product vision is an "easy button" for privacy, an all-in-one privacy app. Email protection is part of that, along with search, browsing, etc. Put another way, we see email protection as part of our core product, and within our app we autofill duck addresses in email forms.
We will take your comment into consideration, though we prefer free services where possible so more people can get privacy protection, which is in line with our mission. This service comes with it a set of privacy guarantees here: https://duckduckgo.com/email/privacy-guarantees and we will not be putting ads on it.