Do they also have a website of their own??????
The reason is that we must pay for the website name (Domain name) and webspace(Hosting for contents of the website) which are not provided for free..
Its not a big deal to have a subdomain name instead of a domain name as they provide you all features for free including domain name,hosting,Templates,site builder etc.
Even you can register your own www.yourname.com domain for the same free hosting website you own ,later. This can be done either by purchasing from your free host provider itself or you can also tag your purchased url to the old site.
List of some free website hosting sites-