Not sure and can't think of why one would use as "BETWEEN -1 AND -10" with "start" as the higher value in any practical case.
Depends on where you are coming from: my initial thought, when I first encountered 'BETWEEN', was "I can't think why one would use BETWEEN in any practical case.".
BETWEEN suffers from at least two ambiguities until you consult the "official" definition to find out what arbitrary choices have been made to remove them.
The first is the issue in question: it would have made just as much sense, if not more, to make the definition symmetrical with respect to the two bounds. Because  it would be more work to write down the alternative expression and  the keyword AND in all other contexts implies symmetry of the operands, unlike e.g. 'x BETWEEN a TO b'.
The second ambiguity is whether BETWEEN includes or excludes the bounds. Intuitively I would make the wrong assumption there as well.
So for me it is a bad language feature, well-deserving of at least one Question of the Day.