For example, with Swift, IBM is a major code contributor and runs significant portions of the project. IBM is one of the largest patent holders in the world. If Apple and IBM ever had a patent dispute involving swift, wouldn't that naturally lead to attorneys scrutinizing every other similarly licensed open source project to find an angle against each other or their partners? Wouldn't that then lead to other companies getting drawn into the dispute? And once the gates are opened, any long-simmering patent issues would be easy to add... affecting more projects and companies...
What would have happened if the Microsoft/Google/Android dispute involved Apache 2?
IANAL, but these boilerplate patent-agreements-in-open-source-licenses seem scarier than just leaving things ambiguous.
A) give some chunk of your profit to someone else while you
B) reallocate development resources to invent something new that lets you keep doing business without relying on the patent
In the end, B) is valuable and A) costs money and they probably cancel out, financially.
The reason it doesn't matter is because of this:
"then any patent licenses granted to You under this License for that Work"
As the owner of the patents, Apple doesn't need this license to have rights to those patents. They have rights otherwise.
Now, it would terminate the rights they have to other people's patents under the license. (IE if a contributor submitted something the contributor owned a patent to, apple would lose rights to that patent)