No, people don't do it ignorantly. It's just when little pissants like you learn about it, you assume everyone else is as stupid as you are, and you feel compelled to continually point out the obvious, as if it's some sort of "news" to the rest of the world, just as it was "news" to you.
Paganism is ancient. Yes we celebrate major holidays on what were once pagan observance days. Who cares? What's wrong with Christians observing solstice and celebrating the birth of Christ? Solstice is a natural event that is guided by the hand of God. Big whoop.
Personally, I love the long nights of winter. And I'm a Baptist who celebrates halloween (though the history of that particular holiday is a little garbled...it might actually be originally a Christian celebration...with pagan overtones. Honestly, it's a 20th century tradition more than anything) and Easter and Christmas and embraces all the pagan symbolism...without attaching any superstition to it. We have a "dressing of the greens" celebration in our church...and I hide eggs on Easter. HORRORS!