If an administrator thinks it's appropriate to block another instance, then who am I to complain if I use the instance free of charge except perhaps donations?
You're completely right there can be such reasons for administrators to block other instances and that's up to them.
It's probably amazing because everybody is used to social media platforms blocking access to and from other platforms. The point of these commercial platforms is to reel in as many users as possible and keep them in the ecosystem. No export possibilities, no federation or standard protocol.
It's like a large company inventing e-mail and not allowing people to e-mail to an e-mail address registered to another domain. Nobody would think that's logical, but most have grown accustomed to commercial social media locking every account in.