> Why are hardware-based manipulations, like black and white photography (traditionally using black-and-white film), long exposure, etc., which also result in an "unnatural" image, acceptable while software-based manipulation (like HDR) is frowned upon by the photography community?

Because we feel we need to put a line that separates photography from painting **somewhere**.
 
Most will agree that straight-from-camera photo of a brick wall constitutes photography. Also, most will agree that [1996 John Corkery's depiction of Hedy Lamarr used in Corel Draw ads][1] does not.

Ultimately, one can take a photo of a something and "keep modifying" it over and over until nothing of the original photo remains and digital artist's vision takes over. The easiest and least controversial threshold is to refer to older times when photography was already established but digital art was non existent.

So to answer your question: **it's a convention out of tradition/inertia/whatever you call it.**

It keeps changing and evolving. What's frowned upon today would likely be mainstream in few decades.

*I intentionally used "lifelike vector illustration of Hedy Lamarr" (actual legal description) as an example of something that is undoubtedly 100% digital art, yet it's based on a photograph.*

  [1]: https://www.google.com/search?q=hedy%20lamarr%20corel%20draw