Why aren't Tweets tagged with #?

Question

Wouldn't be it better to tag keywords (with prepended symbol #) in published Tweets?

http://twitter.com/stackenglish

some examples:

"A lost case, that is." http://ur.ly/CNPR #grammar
"An RPG" or "A RPG"? http://ur.ly/CMRa #pronunciation
When referring to a previously sent text, would you say 'I text you about that' or 'I texted you about that' ? http://ur.ly/COhY #usage
"I'm having" - an illness? http://ur.ly/COfk #grammar
What is the origin of "earthling"? http://ur.ly/CMuD #usage
Should I write"a tweet" or "a twit", "to tweet" or "to twit"? http://ur.ly/CMIL #usage
Word for "someone who does the same job as me" http://ur.ly/CKgA #wordchoice

Why aren't #tagging used in Tweets?

Answer 1

Currently, it's turning the most popular of the given question's tags into a hashtag (as seen here).

If SO wanted to add another hashtag, what criteria would they use? That is, how would an automated system know what to emphasize?