Why is mobile contacts sync a paid service?

We found that supporting a wide array of mobile phones was extremely challenging so we partnered with a leader in the space to develop the feature. This service costs Canonical money to provide to subscribers so we are only offering it to paid plan users.

As part of Ubuntu One Mobile, the first 30 days of contacts sync service are free. If you don't like it, you should cancel within the first 30 days to avoid having your credit card charged. If you do like it, you won't have to do anything. Your Ubuntu One Mobile service will automatically renew.

Ubuntu One FAQs