Yes, the US tax payer did build the airfield and runway at HHN. Uncle Sam did not however pay for the terminal building - which cost a chunky pile of cash.
HHN when it opened as a non-military airport had pretty much no commercial traffic and being in the middle of nowhere is far from major urban centres. Airlines knew it was remote, and thus had littole inclination to fly there.
The strategy chosen was to charge (and still charge) the airlines very low landing fees, to incentivise them to come to HHN, set up a sizable operation and hopefully start basing airframes and crew at HHN. This strategy has worked in that HHN now has a non-trivial amount of commercial flying going on.
However, HHN has been losing money for years.
The owner has a choice - either to subsidise HHN, or to find a way of raising more revenue.
Some rents on shops, restaurants, car rental firms, etc...are either determined by leases with pre-set rents. Other rents are short term - but raise them too fast, and the companies running the shops just decide to close the stores and pay no rent !
AFAIK, landing fees to airlines are not being raised substantially, meaning MOL cannot make a huge fuss on this angle. Instead, pax are being forced to pay more to the airport, so that HHN becomes a profitable airport, and the roof can be weather-proofed.
Yes, it is annoying having to pay the extra 3 euros, but the money has to come from somewhere. HHN is a business, and cannot be run at a loss forever.