It is a bit confusing. Is it a bug or desired feature?
As per the manual -- https://htcondor.readthedocs.io/en/latest/classads/classad-mechanism.html#attributes-- the bug is that HTCondor isn't rejecting ClassAd attribute names with a '+' in them out of hand.
If it is desired, are there other characters causing the quoting?
Presumably anything that's not an alphabetic character (in the ASCII sense, I presume), a digit, or an underscore. '+' is probably particularly bad because it would clearly cause an ambiguous parse in an expression (is it adding two attributes or a single attribute reference?).
-- ToddM