United domains always had the .de domain. People are used to it, it's part of the brand.
Now they have united.domains. What should they do?
1. Resolving both domains top the same website -> duplicate content, bad for search engines
2. Forwarding the .de domain to the new one -> People would be confused about the reason, thinking it would be a new owner of a hijiacked website. And their stored credentials would be lost as well.
So, the only realistic choice is to forward the new domain to the existing one. That's what all sane people do when they get a new matching domain.
They still have the option to switch to the .domains url, but rather when they eventually relaunch their site anyway, or when something else really important changes. But you should never change your accepted domain to a new one without a very good reason.