It might not always be enforced in practice, but StartCom's policy doesn't allow use of the free certs for a commercial purpose:

> Class 1 certificates are limited to client and server certificates, whereas the later is restricted in its usage for non-commercial purpose only. Subscribers MUST upgrade to Class 2 or higher level for any domain and site of commercial nature, when using high-profile brands and names or if involved in obtaining or relaying sensitive information such as health records, financial details, personal information etc.

https://www.startssl.com/policy.pdf