What I meant was that grammar is part of the language and anything written down as a rule is intended to describe how the language can be used to work. The same way that dictionary writers don't invent the meaning of words, grammar teachers don't invent the rules of grammar. The safest test of good grammar is whether something sounds right, not whether a teacher approves you.
You'll see people abuse the logic I'm using, to say things like double negatives are perfectly OK. But its obvious that the reason we avoid double negatives is because they don't communicate things clearly, so we mark double negatives to be an inarticulate use. Other uses of language may be malformed or poorly formed. But, there's absolutely no reason to believe that 'they' can't cover the use I'm referring to; language isn't this inflexible thing where there can't be secondary uses for words. Just because they clearly refers to a plural in once case, doesn't mean it can't be adaptably used to refer to a singular in another case.
Passive tense refers to the verb tense, passive voice refers to the style of writing which uses the verb tense.

And sometimes passive voice is more effective than active voice---passive voice can give more of a sense of objectivity because it distances the grammar from the writer. That's why it was popular in the 19th century. Today, its just a hangup for people with a certain mindset where they're biased to think speaking subjectively is preferable.