The lawyers in this situation had a clearly defined client whom they had an ethical obligation to represent zealously. I don't know any of the particulars, but let's assume their official client was the corporation, and the board, stacked with the VCs, duly voted on this course of action. The lawyers were then just doing their job properly, under this assumption, and serving their actual client, not making a calculated betrayal. The only ones responsible for the VCs' decision are the VCs.

Of course founders could and probably should have their own lawyers advise them in deals with outside investors.