…BMY learned the hard way to include such a proviso after its $60+ bid for IMCL was trumped by Icahn's shopping and LLY's $70 bid.
Exactly right; however, I would argue that BMY ended up doing well by not getting ImClone for $60/sh and thereby having more money to spend on more promising acquisitions.
You are looking from BMY's point of view. Of course they want to include this provision. From MEDX and ZGEN's point of view, they don't have to agree to this provision. They are not desparate for being bought out. They had/have plenty of cash.