I see where you are coming from (I have a pet hate for percentage-based fees of funds under management for financial advisors that are not tied to performance or value added).

If the platform has added value to the post (I find eSteem to be easier to use to post photos from my phone), I am happy for it to share rewards (which for me are generally minimal). I see it more like "tithing" some of my income back into the system. The bitshares ecosystem (originator of the graphene platform on which the Steem blockchain operates) I believe was built on a social contract of giving back +5% to holders of bitshares in acknowledgement of use of its technology (PeerPlays recently honoured this by sharedropping 5% of its token among bitshares holders, for example).

So, I think it is a bit different to your milk example in that it is not the wealth of the user, but the production of posts using its technology that is taxed/shared. Would a better example be a landowner allowing a farmer to graze his cow on his land in return for 5% of the milk produced?

It is all voluntary, however, as you say, and there is nothing stopping using competing platforms that may be cheaper or free.