You could charge for MMM's advice. No doubt there are many others that do (such as a book I saw in the airport: Get Rich Slowly). Not to mention that MMM did profit off this site at one stage, if I recall correctly, but chose to remove ads (for credit cards?) in order to remain ideologically pure.
Explanation one: the author of the linked discussion is operating off the incorrect assumption that a person who does not monetise the value they provide is not providing value at all. This is false, as there is no necessary nexus between providing value and requiring monetary compensation... but it is an understandable error. Further reading: altruism, charity, mentoring.
Alternative explanation: MMM is gaining value in exchange for his work, but in a non-monetary form, by encouraging the development of a better / happier / less wasteful world.
Third explanation: as an owner of capital, MMM is operating a business - one that involves providing financing to other entities, in exchange for a piece of the value they produce. This is an economically productive activity, as it earns a return. If it were not economically productive, why would people pay him to use his money for a while?