So all the reviews that mention the performances given by actors and actresses are misguided - because there is no "performance"? Liam Neeson and Ralph Fiennes did not give a performance in Schindler's List? That doesn't make sense.
The cinema is an audio-visual experience. It's not the performers you need to be worried about, but the impact on their performance as perceived by the other cinemagoers. A performance only makes sense if there is somebody there to perceive it, i.e. it requires an audience too, and that audience needs to be respected in order for the quality of the performance to be fully appreciated.
If a baby cries or vomits at a particular point in Schindler's List, then you've missed that bit - and the rest of the film could be ruined, as it's a linear experience outside of your personal control. If a baby cries or vomits at a particular point in an art gallery, you can change your route and come back to that point later - you can choose to make it non-linear. That's quite different to the cinema.
Despite all of that, the reason this mother was ejected was not because she took her baby into the cinema, potentially ruining the performance for other patrons, but because she took her baby into a 15 film! If she'd gone to a PG or even a 12A, she would have been OK. But a 15 ... or an 18??? Rightly not OK.