Google recently revised their listing guidelines for Maps. They specifically added several sections on appropriate category use.
Why? Well because while you and I are stressing over the details like legitimate business naming practices to comply with the guidelines, the spammers long ago went after the long tail of category search very aggressively to maximize ranking benefit. Here are the specific sections of the new Google Listing Guidelines relating to categories:
When entering categories, use only those that directly describe your business. Do not submit related categories that do not define your business. For example, a taxi company might properly categorize itself as “Airport Transportation”, but it would be inaccurate to also use the category “Airport”.
Also, please use each category field to enter a single category. Do not list multiple categories or keywords in one field.
Use the description and custom attribute fields to include additional information about your listing. This type of content should never appear in your business’s title, address or category fields.
If you look at the search results in Maps (Clean Hotels NY NY) to the left you will see that most listings seemed to have engaged in some form of category abuse.
The “Chem-Dry Cleaners” results provide an incredible visual of field stuffing. Obviously the field length limits are quite high and while it should be easy to recognize the stuffing upon data entry in the LBC, Google seems to have not yet done so.
One can presume that the “Good Hotel” folks, listing spam in its own right from affiliate spammers reservationcounter.com, have used the phrase “clean hotel” as a category.
To a large extent category spam has flown under the radar as it is usually the out of sight, out of mind kind of spam and it is a little harder to spot by someone without more access to the index.
Why Google shows some category listings and not always others is not totally clear. It seems that the category information shows up when the algo is not positive that the result belongs in the result due to the ambiguity of search intent.