When you create a message or an event and share it on your timeline, you always select recipients. These are by default the only people who can see that post. Nobody else. You always can add people if you like to, also later on. Companies have no business with what you post and neither does any other outsider.
When you decide to follow a company you allow that company to read your personal data. Those are quite basic after all. Companies can not read your timeline but you do give them permission to advertise on your timeline. In return you will be offered tailor made news on the topic you selected on their page and you get award points to help you upgrade your HowAbout account. We see this as a fair deal. Once you stop following a company the ad will disappear a week after you unfollowed.