Most popular open source licenses (except GPLv3 and Apache) only give you a copyright license to the code, they do not give you a patent license. In theory, somebody could create some software, get a patent for it, release it under the BSD license or equivalent, and then sue everybody who uses the software for patent infringement.
However, some lawyers believe that the BSD and similar licenses give you an implicit patent grant. The licenses say you have a license to use the software. Since you cannot use the software without a patent license, then giving somebody a license to use the software automatically includes patent rights.
The inclusion of an explicit patent grant means that the implicit patent grant is no longer necessary and therefore no longer exists. So you had armchair lawyers divided into two camps. The first weren't so sure about that implicit license thing, so Facebook's inclusion of an explicit grant was a good thing. The second camp noticed a few problems with the explicit grant, and complained loudly that the explicit grant was worse than the previous implicit grant.
With these changes, Facebook addresses the criticisms of the second camp.
However, some lawyers believe that the BSD and similar licenses give you an implicit patent grant. The licenses say you have a license to use the software. Since you cannot use the software without a patent license, then giving somebody a license to use the software automatically includes patent rights.
The inclusion of an explicit patent grant means that the implicit patent grant is no longer necessary and therefore no longer exists. So you had armchair lawyers divided into two camps. The first weren't so sure about that implicit license thing, so Facebook's inclusion of an explicit grant was a good thing. The second camp noticed a few problems with the explicit grant, and complained loudly that the explicit grant was worse than the previous implicit grant.
With these changes, Facebook addresses the criticisms of the second camp.