It’s worth mentioning that NEW Henle scores do have very flat beams that never cross a staff line; but that the traditional sheet engraved scores from Henle actually do create diagonals with the staff lines on occasion.
It is not a rule that you should never cross a staff line. The new Henle scores are not universally considered an improvement, and I wouldn’t look to MuseScore as the state of the art, either.
Angled beams help to create a better sense of the ‘direction’ of the notes than flatter beams. (I’m sure @John_Ruggero will be along to give further details in due course.)