That is in practice not true. If you leave a fake but glowing 5-star review, no business will challenge it. But if you leave a 1, 2, 3, or even 4-star review, suddenly you're asked to provide proof. Of course, they can legally challenge a 5-star review as well. But in reality they conveniently don't seem to care about those.
Anyway, Germany is probably one of the few places where this happens. The issue isn't necessarily that reviews can be challenged. The issue is that users aren't informed when they leave a review that they may later be required to provide proof of their visit.
I once left a negative review of a very popular touristy business in Germany after a genuinely terrible experience. I included photos and detailed information, yet they still challenged the review, claiming I had never been a customer. Google then required me to provide additional evidence to prove that I had actually visited the place.
What made it even more frustrating is that they challenged the review two years in a row. After the second challenge, I wrote to them that if they continued contesting the review, I would consider it harassment and pursue legal action. After that, they stopped.
What I find pretty shady is that most businesses seem to wait a year or two before contesting reviews. By that point, most people no longer have receipts, invoices, or other documentation. If they challenged reviews immediately, customers would be much more likely to still have that evidence available. In my case, I take photos frequently, so Google accepted my proof and kept the review online.
Ironically, after going through this process myself, I've come to believe that some form of verification should probably be standard worldwide. Requiring reviewers to provide evidence that they were actually customers could help reduce fake reviews. But if that's going to be the standard, it should be clearly communicated upfront, before people submit their reviews.
Another related issue I have with Google Maps is that, at least in my home country, some places have reviews disabled because Google considers them too prone to polarization or controversy. Schools are one example.
Personally, I think that's a terrible idea. I'd rather be able to read the reviews and make up my own mind. Instead, Google, in its infinite wisdom, decides that certain topics are too contentious for users to see feedback at all.
I find that to be one of the worst decisions made by the Google Maps team. Hiding reviews doesn't eliminate disagreement or bias, it just removes information that users could otherwise evaluate for themselves.