This is the relevant part from their T&C:
2.7. Furthermore, we reserve the right to terminate the contractual relationship without notice for good cause.
The full part reads
Furthermore, we reserve the right to terminate the contractual relationship without notice for good cause. Such good cause is deemed to exist, among other reasons, if the Customer fails to meet its payment obligations or violates other important customer obligations. A further important reason which may result in us locking or terminating the Customer’s services or account without notice is if the Customer uses content that impairs the regular operating behavior or the security of our infrastructure or our product, or violates paragraphs 8.1. - 8.3. of these Terms and Conditions.
I think Section 8 "Use of the services / content" is likely more relevant in this case.
Given one of their goals on their homepage is "access to internet ... [due to] ... outright censorship" (which is legally required in some countries), likely they are in violation of
"8.1. The Customer is obligated to check and comply with the legal provisions arising from the use of the contractually agreed services"
and
"8.4. If we become aware of illegal activities, we are obligated under Art. 6 Abs. 1 DSA (Digital Services Act) to request that the Customer immediately removes the offending content and we are entitled to lock the Customer’s access to their Hetzner services or account."