Minpaku is a private accommodation or lodging available for rent – not a hotel, Airbnb, etc. Staying in a minpaku is often cheaper when compared to hotels, and offer more options for guests. Also, the owner of the apartments/houses can earn some money from them.
In regards to this article, the thing we want to communicate is that not all the rental apartments or houses allow you to sublet in this minpaku style. Stay in one, by all means, but before you try to make a few bucks on one yourself you should know that the owner makes the rules. i.e. Keeping a pet is prohibited or smoking is prohibited (if specified in the contract, of course). 99 % of housing contracts in Japan specifically prohibit subletting, and if the Lessee (tenant) violates that clause, the Lessor (owner, etc) has a right to cancel the contract on the spot.