How would this work in court?
"We're charging you with breaking some laws, but we're not going to tell you which ones. How do you plead?"
Obviously, Microsoft disclosed what patents they believed were being infringed when they decided to actually take people to court. They decided not to disclose that when negotiating licenses, which is actually legal. Contracts can include non-disclosure agreements that would prohibit you from telling Linus what parts of Linus to change, and you're allowed to vaguely threaten people as a negotiating tactic, as long as there actually exists some underlying infringement which Microsoft could reasonably have believed to have occurred. (Otherwise, it would be extortion.)
Patents are public information, including what patents Microsoft has been issued, so the law would assign device manufacturers the burden of determining which of Microsoft's patents have or have not been infringed by their products. Microsoft is not legally required to do their work for them and tell them what they're infringing. All of the parties involved could have conducted patent reviews of the Linux kernel or AOSP to find and remove the infringing bits Microsoft was suing over. Instead, they decided to individually settle because the engineering and legal resources required to fix Android's apparent patent problem were less than just paying Microsoft for a license.