Maybe I'm wrong, I don't want to put words in that user's mouth, but my interpretation was that they'd called resort fees are a scam because they only show up after you've booked your stay. That's the scammy/fraudy/dishonest part of the transaction; without it I'm not sure there's anything to complain about.
But I also consider them a scam in practice because hotels & OTAs go to such lengths to hide them during the search process. By advertising a rate that doesn't include these fees they are advertising a rate that simply doesn't exist.
It is (sadly) the default practice in the states to advertise the price of basically everything pre-tax, so at a minimum consumers here expect for tax to be added (though folks new to booking a hotel probably don't know about occupancy taxes). But for a hotel or OTA to advertise 2 different hotels, one with a $30 / night resort fee, as both being $199 / night is also a scam.
Hotels do this for banquet & convention services as well. Everything is "plus plus", meaning plus tax plus a service fee %, usually 20% in my experience. Your $10k banquet hall rental is really $13k if the tax on it is 10%.
The complaint is not that the fee is problematic in and of itself, it's that the fee isn't revealed until after your booking is made, usually not until check-in.