Ah, you found the rule for when you don't use it. Contrary to many textbooks, teachers, and websites, the lack of an apostrophe in "its" is not an exception--you don't use the apostrophe for pronouns. "His". "Hers". "Yours". As apposed to "Jack's", "Julie's", "the user's".
"Its" is only used in places where "his", "her", and "your", which don't have the s's, are used. People don't use "its" in places where they'd use "hers" or "yours". It's awkward.
