Actually, they got that because they're Push-rod V-Twins running aginst inline DOHC fours.
That was more or less a rhetorical question. The apples and oranges thing still makes no sense to me.
With that logic, if Chevy decides to run their Cobalt Ecotec 4 cylinder in Pro Stock, it gets to be 900 cubic inches.
It's a heads-up class designed to showcase each manufacturer's engineering skills. Heck, the Fords aren't competitive in PS, but I doubt that anyone would lobby for them to run light or run a bigger engine - even when the cylinder count is the same. It would be totally embarrassing for Ford and ought to be the same for H-D.
Yes, I know that PS cars have gone astray from that concept, but it's not as crazy as what's happening with the bikes.
If you can't compete with your peers, then engineer something that will. If you're locked into 1910 technology, expect 1910 results and don't lobby for a rules change to make your weak pup a player with the big dogs.
Harley used a clean sheet of paper and does not have a single production part in their engines. If you get the chance to start from scratch and can't hack it, then maybe you're beating a dead horse (or an obsolete engine combination).
What's next? A 400 cubic inch Briggs & Stratton single cylinder?