There are actually four different "dashes" in La/TeX. The hyphen (-), en-dash (--) which is used for numeric rangen like 1--2, the em-dash (---) for punctuation, and the minus sign ($-$). Knuth talks about them in the TeXbook which is good fun.
Not in ASCII. My definition of plain text is roughly "the characters I have on my keyboard". Unicode is like a superset of all possible plain texts. Useful, but I really don't like my own files containing characters I can't (easily) type. If I regularly typed in another language I would acquire a keyboard for that language. I'm not even convinced typographical symbols like various dash types even belong in Unicode at all to be honest. It seems like you have to draw a very arbitrary line somewhere.
It's not too restrictive for me. I rarely need to write foreign place names or words (I'm British). Yeah I use the £ symbol so I'm not limiting myself to ASCII, just what is on my keyboard (I have € too). I just don't really consider a file full of characters I can't type to be "plain text" just because it's UTF-8, that's all.
Wherever you learned ASCII from, it was very wrong. It probably made the common (although less common in the 21st century than in the 20th) erroneous conflation of ASCII and Latin-1, or IBM code page 437, or IBM code page 850.
