So long as SSP's operations don't meet the definition for a prohibited monopoly - which to all appearances they don't at the present time - or until NR's objectives are formally changed, this is the reason that stations are filled with SSP retailers.
We accept that Network Rail has a monopoly on owning stations, we accept that Network Rail has a monopoly in many areas, so I don't know what people on here want Network Rail to do...
a) accept lower leases, in the hope that this will attract some independent places to win bids (even though there's nothing stopping them from bidding as things stand, or for SSP to still bid most)
b) only allow one party to operate a certain proportion of station coffee/sandwich shops, let's say one firm can't operate more than a third? But we know that the idea of spreading train ownership between three ROSCOs didn't drive prices down significantly, it just created an effective cartel. If we care most about the service that passengers get then would this actually make much difference? e.g. the motorway service stations aren't all operated by one firm but they all charge big money at their sites because they know that people have no other option - if you pay £3 for a cup of coffee then people will charge you £3 for a cup of coffee
c) find some new revenue streams to make up for the reduction in leases (given that any Network Rail income essentially comes from the Taxpayers or Passengers, and we don't want either of these to have to pay more just to save people a few pence on a "Cornish" pasty
Maybe Grant Shapps will be reading this and his next distraction tactic will be to suggest that "wouldn't it be nice if cafes on train stations were more like Farmers Markets, and you could have locally sourced organic foods with lashings of ginger beer, just like in t'good old days" - that'd generate a lot of favourable coverage from blowhard newspaper columnists and keep his boss's shenanigans off the front page for a news cycle or two!
It's funny how the mood on here seems to be that we should have one provider of trains, with a "British Rail" that operates everything, but we can't allow one firm to dominate most (but not all) coffee shops on platforms... for me, I'd be more worried about the monopoly power of the former, rather than focussing so much on who sells you your coffee!
Network Rail is not obviously doing a good job of reducing its dependence on subsidy in any case.
What do you expect them to do though? They have these sites that they can lease - they have to bring in some revenues to counter the billions of subsidy that they require