If you don't do anything legally threatening, then you make it that much harder for every single OSS vendor to make money, because the precedent is getting established that there is no penalty for breaking the rules.
When I was a teenager I would do super cut-rate work on computers for people, and my father did helpfully point out that undercharging for valuable work just makes it harder for people whose day job is to do the same work, because then they have to compete with a naive teenager. You're the kind hearted OSS / freemium vendor in this case. Threatening legal action costs nothing. Punishment is meant as a deterrent for antisocial behavior. Failing to even threaten them will result in less money going to people who deliver a public good.