The problem is not the registration, the problem is that BNetzA considers software to be "telecommunication services" and thus subject to regulation. There are 152 paragraphs in the TKG (https://www.gesetze-im-internet.de/tkg_2004/index.html), how much would it cost a software vendor to comply with everything? Or how much would it cost even to find out what a software vendor would have to comply with? There was a pretty good reason Google went to court over the question whether GMail is a "telecommunication service" or not (https://www.noerr.com/de/newsroom/News/gmail-ist-ein-telekom...). Google lost, by the way.
From the other hand, the list of "providers" includes a lot of organisations like "Antennengemeinschaft Schreiersgrün e.V" - roughly translated as "Antenna community of the village Schreiersgrün (registered society)".