Here's why:
* Free writing is about getting ideas down. It's a stream of consciousness exercise, so the focus is on generating ideas, exploring thoughts, and letting your creativity flow.
* Grammar can be a barrier. Focusing on perfect grammar can hinder your thought process and slow you down. It's better to let the words come out as they are, without worrying about mistakes.
* You can always edit later. Once you have a first draft, you can go back and polish it up, including fixing any grammar issues.
In short, free writing is about getting your ideas out on paper, not about perfect grammar. Focus on the content, and worry about the grammar later.