In my posts above, I was referring specifically to names of mountain ranges that include “Mountains” or “Hills” (i.e. plural words) as part of their name.
There is also, for example, “The Alaskan Range” in Alaska. You should use a singular verb with this particular mountain range. Its name is singular.
Could you post the link to the web site(s) where you saw the unusual “singular verb” usage? I honestly think you must have read something wrong. I haven’t been able to find any examples at all — not even using “The Rocky Mountains straddles the border” (from your original example) as search words in Google.
What I did find using the search words mentioned above was the following sentence, for example:
The Waterton-Glacier International Peace Park in the Rocky Mountains straddles the border between the Canadian province of Alberta and the US state of Montana.
The reason that the singular form “straddles” is used here is because it refers to a singular subject —> “Park”
(Rocky Mountains is not the subject of the sentence above!)
Regarding the word “police”:
This is a “special word”. “Police” is a plural word without an “S” — just as the word “people” is plural — and you should use the plural verb form. Saying “the police is” sounds “uneducated”. When I think of “police”, I don’t picture just one person. (Just one member of the police would be a policeman)
I’m sure some native speakers will use the word “police” as a singular word sometimes. That is nonetheless “a grammar mistake”.
Singular words/expressions would be, for example: “policeman” (one man) or “police force” (one force).
Of course native speakers sometimes say things in incorrect ways (e.g., because they’re thinking something else in their heads, or they’re using an informal slang expression), but this doesn’t necessarily mean it’s correct or even typical.