Full device fees on new contracts

I took out my upgrade on 25-Oct-17. I received my bill for Nov-17 and have noticed that only my data package was billed pro-rate but I was charged for the full month handset fee. How is this possible when I only had the phone in my possession for 7 days. I made inquiry via the call center billings department and they have advised that whether you take out your contract on the 1st, 25th or 31st of the month, you are liable for the full month device fee. How is this legal???