I can think of a few reasons for the grammar rules in the commit message...
It is direct, concise, consistent and tends to active voice rather than passive voice. Further, not everyone writing or reading the messages will be a native English speaker. Avoiding past-tense forms just makes it easier.
I am actually not a native English speaker and I always thought that this was hard to understand specifically because English is my first language. But again folks provided enough information for me to understand the reasons for it to be this way.
