StartCom does allow their free (Class 1) certificates to be used for commercial purposes. They just reserve certain features that many businesses would appreciate (like having your business name on the certificate) for paid certificates (Class 2 or higher).
From their FAQ:
"2.) The certificate is for my company, what shall I do?
In the Class 1 settings (free), the only possible relationship between StartCom and the subscriber is with individuals, i.e. natural persons. StartCom has no relationship with the organization a subscriber may represents and acknowledges only the subscriber. All responsibilities according to the StartCom CA Policy are that of the subscriber personally, even in case he/she decides to obtain certification as an employee or representative of an organization.
Organizations should perform Class 2 validation and an organization name may only appear in a digital certificate at Class 2 level and higher." [0]
From their FAQ:
"2.) The certificate is for my company, what shall I do?
In the Class 1 settings (free), the only possible relationship between StartCom and the subscriber is with individuals, i.e. natural persons. StartCom has no relationship with the organization a subscriber may represents and acknowledges only the subscriber. All responsibilities according to the StartCom CA Policy are that of the subscriber personally, even in case he/she decides to obtain certification as an employee or representative of an organization. Organizations should perform Class 2 validation and an organization name may only appear in a digital certificate at Class 2 level and higher." [0]
[0] http://www.startssl.com/?app=25#2