It's not always possible to eat before/ after due to the timings of films. For example - recently bf and I went to see a film on a midweek eve. The showtimes were 1830 and 2130. It was a slightly over 2hr long film - so we opted for the earliest showing as otherwise we would've got home too late.
Now as we both work ft, we couldn't get to the cinema before 1815. So no time to eat dinner beforehand. And we wouldn't have got out til nearly nine which is too late to eat - especially as bf had not eaten since11am and I'd not eaten since lunch at 1230. So we nipped into the local Tesco, bought a sandwich meal deal and took it in with us.
Unsurprisingly, loads of people in the cinema with us had done the same, or else paid 3x as much for the hotdogs and nachos they'd bought in the cinema.
In terms of time 'at' the cinema, it's oversimplistic to refer solely to the duration of the film itself. You have to travel there and back, which could be up to an hour each way depending on where you live/ if you go by car or public transport, and also the start time as I mentioned. It's quite easy to see how it can be impossible to eat before, and then too late to eat after, by time film has ended and you've got home.
If cinemas allow consumption of their own food - which they clearly do, indeed encourage (and not just cold food but hot food as well) then I don't see any reason for people to get all uppity about people bringing their own food in! I totally get why cinemas discourage this, because of the money they make on food they sell, but why is it such a big deal to people if someone is eating in the cinema or not?
Fwiw people have been rustling sweet papers in theatres for years - I sat behind a lady in the Barbican when I was at school (c 1988/89) who during a Shakespeare play noisily ate a quarter of mint humbugs - unwrapping and rustling each one. We also took our lunches to more than one theatre trip during A levels when our coach had been delayed en route and we'd not had chance to eat. No complaints were made, and no incidents ensued!