My wife (most definitely NOT a Christian woman) does that at Starbucks. I probably would, but I don’t often find myself at Starbucks unless under orders from The Missus.
When life gets really stressful for her, she is more likely to go to Starbucks for a treat. And when things are particularly hard, she’ll pay for the person behind her. It’s her way of flipping the bird at the universe when it seems to want to crush her into a grumpy, selfish person. And of course any time someone buys hers, she pays it forward as well. I would, but it just doesn’t come up.
ETA: For what it’s worth, it’s not about creating any sort of obligation. It’s about you receiving a random act of kindness, and it’s about creating some very low-level social interaction between strangers who ought not to be strangers. If it bothers you that you feel obligated as a result, then you need to reassess what you think a “gift” is. And if it really sticks in your craw that the money wasn’t donated to charity instead, then make the donation yourownself.