"They are painting the house."
Translation:He maalaavat taloa.
It's because verbs in the 3rd person are ambiguous as to what the subject is. With maalaavat it can be he or ne. We don't know which unless it's introduced with a pronoun. Verbs in the 1st and 2nd person already tell themselves what the subject is (for example maalaamme can only refer to me) so the pronoun is optional.
An easy way of determining whether a verb is a partitive verb or not is by asking yourself the following question: Is it resultative? If it's not, then it's a partitive verb. The act of painting can reach a state of completion so it's resultative. Partitive case is required for the object of a resultative verb only if the action is ongoing, because then it's not completed yet.