To cover operating expenses a crypto currency network must charge transaction fees.
Why? If usage of a cryptocurrency network generates extra profit itself then fees can be as small as it's enough for fighting spam. Hashcash could be used to remove fees completely.
Someone has to be paid to validate blocks and run the network. How they get paid and who pays them is something else entirely. I suggest that a system is not sustainable unless income from some source is greater than what is paid to block validators which is greater than the cost of validation. How does a network generate extra profit itself (aside from appreciation) without fees. If appreciation is how it accumulates value, then it would require dilution to pay fees. If you expect some users to subsidize others then that is a group trap and depends upon charity, especially a scale. Uhhhhm, If I run a Ripple node whether I process 1,000,000 transactions or 1, the fees paid to me will be exactly 0. Yes, people making a transaction have to spend a fee, but those fees are all burnt and do not go back to the nodes. So yes, there is at least one way to make a blockchain run without someone being paid directly to validate blocks on a per transaction or per block rate. As CfB mentioned hashcash is another. I also know a dev that has been working on a third model where no fees are paid, but instead people running nodes will get special privileges in the system that others won't. One who isn't just thinking about money all the time just need open their mind to find lots of possibilities. Here is an example. Lets say I am running an anon coin. I can make a rule that only a node that has been active on the network for the last 24 hours can actually send anon transactions, all other nodes on the network less than 24 hours can of course still send and receive, but can't send anonymously. Here is another example, lets say I run a network and anybody that has been running a node for 24 hours will get 10 tokens. Each transaction costs exactly 1 token. All tokens are non-transferable. So now to use the blockchain a person needs to run their node every once in a while for 24 hours and then they get 10 free transactions. I call that proof-of-participation. It is fairly easy to design a system that isn't just focused on money.