The websites are restricted after the license expires.
Why the restriction exists:
Hosting companies asking to protect them from losing customers, as people can switch to a cheaper hosting provider after a website is created;
But when websites stop working (on another provider) customer usually returns back to the old provider;
This model fits all servers network architectures (1 big server; many servers; every customer on dedicated VPS);
There is no other way to control the number of websites as hosting providers constantly migrate among servers;
Legal side (most important). Site.pro uses hundreds of patents, technical suppliers, and content providers; some of them require constant payment while technology or content is used. If Client converts websites to Wordpress (or other technology), its infringe the legal rights and automatically would generate legal issuess with Site.pro or technology or content owners (or their representatives).