Why can't I keep my Armed Forces discount when also trading in a phone?

[Chazlamb]

Today I tried to add a line to my account. I wanted to purchase a new Samsung galaxy z4 flip. The initial contract price was £59pcm. 

I was asked if I had an old Samsung phone to trade in, to receive a discount on my line rental. I did, and after entering the model details the line was reduced by £16pcm. Great. 

I am also a member of HFM , and eligible for a 20% discount on my line. (as an aside I also qualify for multi line discount, 20%). When I added this on it reduced my pcm to around £37. 

All great. But when I tried to order it said I was not eligible and had to call in. When calling in I was informed that (it seemed) I couldnt "stack" discounts and could either have my HMF discount or the trade in discount. What's the logic here? I thought the trade in was essentially a buy back as I'm giving EE a phone, and they reduce my monthly costs. Then being told that if I do so, I have to waive my other loyalty / HMF discounts just seems bizarre. Aren't these my rewards for being a loyal customer? Why should they be removed for the Privilege of giving EE my old phone? 

[Chris_B]

@Chazlamb  As the tariff is reduced by you trading in you can not have a 2nd discount added to that tariff.  You can only have 1 discount per tariff.  You can have the tariff at full cost and then add your discount or you can have what’s offered without your discount.    You can still trade in and have your discount and any monies you receive for the traded in device will be but on your account for future bills or you can have the cash back but you can not have the reduced tariff for trading in also.