This is really sad: I saw Eliza's post on Facebook about this. Presumably the ignorant person who told her to stop did not recognise Eliza and hence how much this would mean to her.
Yes, on other occasions it can be irritating to have someone next to you singing along all the way through a concert, especially as, unlike Eliza, they are usually way out of tune. I have been known to do it, but only if the performer asks the audience to join in, and that's usually only on any choruses.
And beware if you try anything like that in Edinburgh Playhouse during a live concert: in recent times over-zealous stewards have been ejecting patrons for such heinous crimes as singing along, tapping their feet (or hands on knees) or clapping to the music!!