Its tricky to stop the free tickets though. Usually the nix have got some benefit through sponsorship and the tickets are an easy way for the nix to pay for that. The dom post would give nix free advertising in exchange for the tickets for example. Stop the free tickets and you have to pay for advertising.
They should try to limit them where they can though. Reading cinema gets tickets in exchange for players getting free movie tickets (and probably other things, I don't know the ins and outs) but as an example maybe the nix could work into the deal that the tickets must be sold as part of a package deal, rather than just given away as freebies. The cinema can still get their usual allocation of free tickets, but may also request to purchase more at $X if the package deal is popular above the free allocation. That way they keep the sponsorship, they've found another outlet to sell tickets from and it doesn't devalue the nix product in any way. the cinema gets added value of a bargain deal to sell (and maybe take a wee commission from every ticket sold too - as well as actually selling the free tickets instead of having them sitting in an office cupboard doing nothing)
Disclaimer: This is all hypothetical btw. I may work there but I don't know the ins and outs of any sponsorship deals.