Johnny,
UPS doesn't do much of anything consistent nationwide as they originally had piecemeal contracts all over the country with regional negotiations with the Teamsters.
While we have been under a national contract for decades now the regional supplementals still hold reign in many many articles of the contract procedurally.
In my area of the country they can have seasonals both for summer vacation coverage and Christmas help although they cannot repeat use the same people in the same year.
Possibly you are mistaken or it is different in your regional supplement.
Sooo many things are different throughout the nation.
Even the hiring rules are governed by supplemental langauge.
In my area the hiring rule is six inside for every one outside hire, but UPS has the right to use all outside hire for seasonal driving use if they want.