With linking words like "and", "but", "so" and "or", I believe the comma differentiates from linking phrase to phrase (no comma) and clause to clause (comma before the linking word - signifying the clauses either side are being linked rather than two smaller phrases within those clauses).
But the issue here seems to be that clause to clause links have been made, but the second clause in each one does not have a subject mentioned. Have the subject pronouns been dropped in the second clauses?
Interesting!
- Jason