They do. Our single-domain SSL certificates cover both the www and non-www variations of the domain name.
Knowledgebase
Can I install one PositiveSSL Wildcard certificate on multiple servers?Positive SSL WildcardPositive SSL Multi DomainPositive SSLI have multiple domains and subdomains. Can I use Multi-Domain SSL? Will this certificate also be used for subdomains?How do I create a CSR to activate an SSL certificate?What is the difference between a Wildcard and a Multi Domain SSL?Do your basic SSL certificates cover both domain.com and www.domain.com?Is there a limit to the number of subdomains your SSL certificates can protect?What is an SSL certificate?