When were the investments made? Which of these VCs (if any) has a controlling position on the Board? It's not like a VC can say "Oops" and demand the money back after a company turns unethical.

That said, writing to the VCs might indeed be an interesting strategy for handling ethically iffy startups. In fact, probably the most effective I can think of offhand. I'd just strongly recommend a polite "Please look at what this company in your portfolio is doing... I respect your group and I know you'll want to talk to them about this", not a "You! You're to blame! Damn profiteering bastards!"