Earlier quoted context omitted.
In Germany its a cultural thing. People just prefer the option of privacy.
Then people should be free to use cash, but there's no need to lock out people who prefer card payment (I also prefer cash, but mainly because I'm old-fashioned). But if a restaurant has a sign in the door "no card payment due to technical problems" for years, it's probably neither a technical nor a cultural/ideological issue ;)
There's also the parallel EC Card system in Germany, most ePoS terminals only support normal cards, or EC card.
Its easier to just accept cash. Everyone who lives there knows cash is king, and the tourists figure it out within a short span of time.