It is long standing tradition in Unix tools and languages to ignore
the backslash on non-special characters, right from C on up.
Ordinarily, I'd agree, but in this case I think it's right to warn
about it.
In awk one often stuffs regexps into string constants, and it's easy
to forget that if you're trying to escape something *in the regexp*
you need to write two backslashes. Then one ends up with expressions
like (foo ~ "\.c$") that superficially look ok but silently do the
wrong thing. This is doubtless why mawk behaves the way it's reported
to, but I don't think adopting that behavior is a good idea.