Emotionally I have always wanted to believe the learned behaviour theory, but I have kept on so many pets and abandoned ewe lambs and seen them become cracking mothers themselves, I can't say that my experience backs that theory.
What I do think is that any ewe placed in what feels to her like a very unnatural situation is quite likely to react by leaving the lambs and trying to save herself.
So the very penning and handling that we do with the first time mums of itself can create the conditions for her to not manage her first mothering well!
But unless the ewes are very fit, and the weather is superb, I wouldn't want to risk leaving first timers alone outside to do it, either, even though I do believe that to be best for them.
I think any mammal is entitled to find the nibbling questing mouth around the udder very strange and not entirely nice at first.
Very often with first timers, it's just a bit of constraint and support while lambie gets its first bellyful that's required.