Why do I have to register (become a member)?

Short answer - You don't!
But - to see All of the site, contribute content, view members' profiles, yearbooks (if any), communicate with members, buy products, earn points - *and to Sign-up for Reunion Events* - you'll need to register. See Why have this site? video.

Although it's mainly to protect member data (as yours will be), maintaining a "Semi-private" site also eliminates spammers joining just to post "junk".

A "side" benefit of registering is for "uninterested" classmates. Registering removes your name from the missing or "lost" list, so other classmates will stop trying to "find" you.