The way the venue would probably argue their case is this:
"We're not the promoter of this concert. The promoter has simply hired the venue and that's all he's paid us for. However that promoter then requires people to sell tickets for him, with those ticket sellers making their money by adding on a booking fee. So Ticketmaster are selling tickets for the concert and getting paid for doing so by adding on their own fee. Similarly Ticketline are selling tickets and getting paid for it by adding on booking fees. We're doing exactly the same thing and we don't see why we should be expected to do it for free when everyone else gets paid for doing it".
Whether that's a valid argument is, of course, a debatable point but the practice is neither uncommon nor illegal:
https://conversation.which.co.uk/travel-leisure/ticket-booking-fee-ask-which-concert/