I have no idea why the people who write math papers make the choices they make. But I'd like to point out that in your second example the parentheses are neede

I have no idea why the people who write math papers make the choices they make. But I'd like to point out that in your second example the parentheses are needed because infix notation is inherently more complicated. In the first example however, the notation is far simpler: a simple list of function plus zero or more arguments.