Against what does the Apache 2.0 patent clause protect?

Question

The Apache 2.0 patent clause says:

Subject to the terms and conditions of this License, each Contributor hereby grants to You a perpetual, worldwide, non-exclusive, no-charge, royalty-free, irrevocable (except as stated in this section) patent license to make, have made, use, offer to sell, sell, import, and otherwise transfer the Work, where such license applies only to those patent claims licensable by such Contributor that are necessarily infringed by their Contribution(s) alone or by combination of their Contribution(s) with the Work to which such Contribution(s) was submitted.

...

If You institute patent litigation against any entity (including a cross-claim or counterclaim in a lawsuit) alleging that the Work or a Contribution incorporated within the Work constitutes direct or contributory patent infringement, then any patent licenses granted to You under this License for that Work shall terminate as of the date such litigation is filed.

What does that serve to protect? As Apache 2.0 isn't a copyleft license, I can license your code and re-distribute it as a proprietary patent-encumbered product.

Is it to ensure that the original author is giving up his patent rights? Then why not put it outside the license and let the license be GPL2 compatible?

Answer 1

Basically, this is designed to protect against using the patent system to circumvent the licence terms.

The first clause means that if I publish anything under this licence, the users are assured that I will not sue them for breach of my patents, as an implicit licence on these is included.

The second clause revokes that right in case someone creates a derived work that is covered by one of their patents, and then proceeds to litigate against their users, effectively adding a fee on the use of open source software.

(non-lawyer interpretation) It would not strictly be necessary to add the second clause, as derived works would also include the implicit grant of all related patent licences. However, in practice end users tend to cave and pay up in this case, and this clause allows upstream patent holders who might have larger coffers to enter the case.