Yes, that's a good question why they don't want our (not living in Germany) money.
I think the background is:
* Paypal is not very common/popular in Germany so most local transport agencies don't bother
* with SEPA direct debit which is the common payment in Germany you can easily initiate a chargeback after you have already used the ticket. Of course that's fraud, but if you live abroad, the effort to sue you would be tremendous
But the same publication says that only a bit over 50% of the population even own a credit card. And debit cards usable online are still a niche product.
So in Germany, can you pay your Paypal bills any other way than by credit card? Honest question, when I lived in Germany the last time, Paypal did not exist yet.
Germans are usually very aware of hidden costs and credit cards are usually more expensive per transaction.
Germans also usually are much less in debt than for example many US citizens - for example the University education costs much much less. Young adults have 1400 Euros (plus!) on their giro account and it is typical to pay from that account.
The usual method is by SEPA direct debit (initiated by Paypal) but you can use a SEPA transfer as well to top up your Paypal balance and pay with that.
Ah, I had no idea that Paypal would do SEPA direct debit . They don't offer it to me. (I don't have a German address, although I would have a German account.)
I think the background is:
* Paypal is not very common/popular in Germany so most local transport agencies don't bother
* with SEPA direct debit which is the common payment in Germany you can easily initiate a chargeback after you have already used the ticket. Of course that's fraud, but if you live abroad, the effort to sue you would be tremendous