[RhysU]

Because "minus" is the boolean subtraction operator not the unary negation operator despite a lot of colloquial misuse by native speakers.

The distinction is useful when reading aloud expressions like 3-(-2) as "three minus negative two".

[osrec]

Perhaps, but "negative one" would sound a little odd to lot of people from certain countries, including the UK, where "minus one" is the standard way of referring to a negative number. We would be perfectly okay with hearing "three minus minus two" in mathematical context.

[RhysU]

Americans would be OK hearing that too. It's just a more complicated grammar to parse.

Curiosity: How do you call the terminals on a battery?

[osrec]

Positive and negative for battery terminals.

But "minus five degrees" for temperature ;)