1. People actively search for "free shipping" in Google - especially around Christmas.
You can bet on customers appending searches with "free shipping" - like "iPad free shipping" - so including it in your title tag will help you rank very well for these queries.
2. Searchers scan title tags in search results pages. Numerous consumer surveys show that customers looooove sites that offer free shipping - they prefer sites that offer it (even when the site has higher product prices). Including "free shipping" in the title tag may win you the click (which hopefully turns into a sale), even if you're not the first or second search result.
Looking for other ways to show off your shipping offers (or other value props)? Check out
Free Shipping: Got It? 10 Ways to Flaunt It