Although it looks like VMware has only one site, there are actually more than one. The VMware Store (store.vmware.com) is another site than the VMware Communities. That is why you have to login again. I agree that it is confusing. You probably need to create a new acount for the VMware Store. If you use the same username and password as for the VMware Communities, you don't have to remember two accounts.
VMware made an effort to combine some websites with My VMware. But unfortunately not all websites are included yet.