I think that questioning the motive was justified.
No, a pretty dick move to piss on her for asking to reconnect with an old schoolmate with a rude response.
FB is not some "personal circle". People connect to wide circles of accountancies they barely meet once in a decade and old schoolmates from way beyond and such all the time. And if one doesn't, it's still perfectly fine, given the widespread practice, for others to assume that it's ok for them to ask.
No other motive needed other than we're all humans, and we tend to think of people that passed through our lives in some way or another, and might want to occasionally catch up and/or share some old-time story.
No further need to "question the motive" (as if she's some huge scammer) than to question "high school reunions".
In the end, one could just ignore the request, or reply politely ("Sorry, I only add close friends and relatives on FB").
There's nothing nefarious about it. If I got your question, I'd be torn between just saying "just curious what you have been up to for ____ years, that's all," and her response, because the question in and of itself seems kind of unusual.
It's not like you need to have some huge reason to want to connect to some people on FB. It's not about being some big taskforce or goal-oriented group of connection.
Sometimes you just lookup people that were in you life to catch up, and see what they're up to. Or to make sense of what your life was, and the people that might have influenced it, or took part in it.
As I wrote elsewhere here, I don't think FB is any real platform to affect change or share opinions (it's at best an echo chamber, at worst nobody cares).
But for casual chat, it doesn't hurt to add some "girl from elementary" school or random acquaintance in your Friends list. It's not like they have to justify their presence in utility. One can just ignore their posts.
The final straw for me was actually a girl from my elementary school who wanted to friend me. The chat went like this...
Me: I have to ask you something. So we haven't seen each other for decades, the last time being when we were 7 year olds. Back then we probably exchanged less than 100 words. I don't mean to be rude but I'm genuinely curious to know why you want to connect with me?
Her: Oh I'm sorry. Didn't mean to offend
And that was it. She didn't have a reason other than "just because". That was when it really clicked for me what an utter waste of time Facebook actually is.
Never looked back