well they need to invest 0 to use the platform. no one asks for anyone to buy hive to use hive. so censorship free for free.
there are few reasons to power up. one is to have influence on governance, and in reality if you are not a big stake holder you don't have to much influence. if you have 100+ small stake holders you could have some.
second is to participate in distribution of Hive. if you go really simple upvoting is kinda like mining. can you mine and not invest a lot to a miners equipment? will that hardware go up in price? your miner on hive could go up in price.
and something that not a lot people think about. imagine someone gets your private key from a btc address, how long will you have that btc?
if you f-up and someone gets your active key, how much hive can they take from you?
it is a perspective thing. if you only wait for a pump to dump all of your hive, than you can just keep it liquid. you can even always be in powerdown. (i am not 100% sure but you can still use all hive powered up until it becomes liquid)