Why... acceptable while... frowned upon by the photography community?
The context changes what is acceptable and what isn't. The context of your original question was "traditional photography rules" not "photography community". But what is photography community? Everyone who owns a camera? There are so many subgroups with contradictory preferences that it doesn't make sense to ask about the preferences of the entire group as a whole.
Nature photographers are expected to photograph nature, not caged animals or taxidermy specimens. Documentary photographers are expected to represent reality as nearly as possible, not to airbrush people out of photos. No such restrictions apply to ordinary people.
In my previous question someone pointed out that unnatural HDR based toning will get more objections... than cropping.
By the numbers, more people will object to the HDR toning than to cropping. Those who object to cropping are likely to object to HDR toning. Many people who find cropping acceptable object to HDR toning — X + Y > X.
Your previous question was about "traditional photography rules" and cropping. While cropping has a long history and can be justified as falling within the bounds of "traditional photography", HDR toning is a recent development that is far less likely to be considered more "traditional" than cropping.
While there are darkroom techniques to combine multiple exposures and increase dynamic range, HDR toning has a distinctive look and modern computational requirements that set it apart from darkroom work or programs that simulate darkroom processes.
It's very much like looking at impressionist paintings. The execution is at least as important as style choice. Some nice images have been created using HDR, but the effect has been over-used. Many people are now tired of it. There are also images where HDR is not well used. This includes images with lifeless colors; halos; desaturated, low-contrast shadows; harsh, blurry transitions between light and dark; and exaggerated, over-sharpened details.
Hardware vs Software
Why is hardware based manipulations... acceptable while software based manipulation... is frowned upon...?
Restrictions to limit photography to in-camera processes might have originated as a guideline for documentary photographers to reduce the temptation to edit their photos. For some, it's a point of pride to get it "right" in camera. For others, it's a time saver to not have to post-process.
As others have mentioned, the distinction between "hardware" and "software" isn't clear because modern cameras are specialized computers. Most cameras have multiple toy modes, and some cameras have in-camera raw processing, along with other editing capabilities.
mattdm speculates: "Differences from human perception which are due to limitations of the medium are generally more accepted than intentional alteration free from that." The dichotomy is natural vs unnatural, in and for a particular medium, not hardware vs software.
Eye of the Beholder
I personally feel that slight HDR based enhancements are OK from an artistic perspective...
You can do whatever you want and call it art. You don't even need a camera. Whether other people agree with your "artistic" tastes is a separate issue.
When people are concerned more about a technique than they are about whatever effect an artist intended, that piece has failed as art. HDR toning tends to be so glaringly obvious and ugly that it draws attention to itself before anything else can be considered, causing the image in which it was used to fail as art.
... as long as the final image is not changed drastically.
Where did this requirement come from? Is it no longer art if the image is "changed drastically"? Regardless, HDR toning does change the image drastically because every pixel is modified in the process.