It is not intended as a heading. It is informational only. For example, above each comment is a username and time stamp information. This is just a statement of information, short, direct, easy to digest information. It is not a complete sentence completed with proper capitalization and ending in a period like, "Here is a comment from husker posted on May 9th at 9:00 AM." It is a simple statement of the facts.
So, having a sentence like "There are 25 Comments." creates a couple of problems. 1) It is structured as a sentence, as if it needs to follow grammatical rules by starting with a capital letter and ending in a period. But the word "comment" is capitalized within the sentence as if it is a proper name, but it is not. (This is not German where common nouns are capitalized.) Therefore the attempt to use a sentence structure kind of fails when used in this way, because clearly they want the word "Comments" to be capitalized because it is important, yet putting it in a sentence structure forces them to break the rules. Note that article titles do not have periods at the end because they are not sentences and are not subject to the same rules.
2) It creates the problems of requiring different verb and plural usages depending on the number of comments.
Having it simply represent a factual piece of information in a succinct way is really the best design choice to make from a both the programmers and end-user point of view.
Number Of Comments: 25
Seems like a clean, clear design choice to me. But hey, to each his own.