That is strange and interesting as it seems counter-intuitive since a B.A. in say, physics, is less rigorous than a B.S. from a pure physics/mathematics standpoint.
That depends a lot on the issuing university. At some of the medieval university, e.g. Cambridge, the only undergraduate degree is a BA, regardless of subject. Likely because the naming of the degree predates the modern usage of the word science.
Few would argue that a BA in physics (which would actually be in Natural Sciences) from Cambridge is "less rigorous"
Cambridge and Oxford and a few others also then grant students an MA more or less automatically¹ a handful of years after graduation², a process sometimes referred to jocularly as the "MA for FA"!
Sure, that's "old-fashioned" as a friend once told me (who was a Rhodes scholar.) At the two universities I've attended, the B.S. in physics required more rigor and the B.A. offered more breadth and didn't require as many courses starting in "PHY". I'm guessing it's an American thing too.
Few would argue that a BA in physics (which would actually be in Natural Sciences) from Cambridge is "less rigorous"