Why does this sentence use imperfect subjunctive tense?

I’m reading ‘Harry Potter y la Piedra Filosofal’ as all Spanish students do.

I’ve come across this sentence: “Dejó que Harry viera televisión”

Am I right that ‘viera’ is imperfect subjunctive? Why is this? I thought subjunctive was used when something hasn’t actually happened or something that isn’t a being said as a statement of truth.

In English: “She let Harry watch TV”, I can maybe see that because it’s “watch” and not “watched” which obviously wouldn’t make any sense, I can see it’s not obvious grammar here.

If I was to say this in Spanish before reading it, I think I would have gone for: “Dejó que Harry ver la televisión”.

Why is ‘viera’ used? Can anyone explain what I’m missing from my understanding of subjunctive tenses? 🦄