On that line you have one concept. Create a new object containing the results of parsing a specific NonTerminal. Now you want to split that single line into a block of statements. Yes, it makes it more readable, but here I think the bigger picture is more important. Adding more than one line doesn't help readability when you are looking at the overall structure of the code.

I'm not sure we can come up with a general rule that can satisfy all of these concerns.