I am pretty sure that you can upvote most of the stuff on instances you don't have an account for. However some instances require you to apply for an account rather than just singing up (eg Beehaw was like that for me). An admin has to manually click a button for you to become a member. In those cases you can not just upvote or comment on one of those instances without an account.
I think it's to prevent a flood of bots on some of the instances.
Not entirely sure if I got it right, though!
I am pretty sure that you can upvote most of the stuff on instances you don't have an account for. However some instances require you to apply for an account rather than just singing up (eg Beehaw was like that for me). An admin has to manually click a button for you to become a member. In those cases you can not just upvote or comment on one of those instances without an account. I think it's to prevent a flood of bots on some of the instances. Not entirely sure if I got it right, though!