I would think that a baby under, say, 12 months, wouldn't cause a problem for anyone in a cinema. And if it did, and spoilt the film for anyone by crying or fussing, then it should be ejected..along with its parent, obviously.
Anyone who creates a disturbance in a cinema would be asked to leave I would assume, no different for a baby. And at under 12 months they're not going to be affected by the content of the film are they? If the parent thinks the noise level is acceptable then that's fine, I think it's fine too to be honest.
It's not something I would do but I really can't see a problem.