Why is a patent grant worse than no patent grant?

The dispute by the Apache Software Foundation over Facebook's addition of a patent grant to the BSD license rumbles on. Corporate-backed Apache developers are livid that Facebook will withdraw this grant in the event their employer attacks Facebook.

A common question “why is having a patent license until you commence litigation worse than not having one at all?” The answer is subtle. Many corporate lawyers operate on the assumption that all open source licenses that do not mention patents (BSD, MIT etc) implicitly grant a patent license. Clarifying this ambiguity is seen as harmful — that’s why approval of CC0 at OSI was abandoned, for example. Including an explicit patent grant removes the possibility this could be argued in court and is seen as an escalation of the patent conflict by Facebook.

Given many voices at Apache are being quietly guided by corporate counsel, this seems the most likely underlying explanation.

Much more on the Apache vs Facebook situation at:
https://meshedinsights.com/2017/07/16/apache-bans-facebooks-license-combo/#implied