There is no reason.
The costs are visible to everyone so you should do what's best for you.
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Well in this case it meant that they lost out hugely.
Samsung offered me a 15% discount on the handset, so I took them up on that. My wife will probably do a similar thing and buy elsewhere from a company that actually means it when they offer a discount to key workers. Then we will both claim the blue light discount on our existing SIM-only contract (£16 versus the £23 they wanted us to sign up for with an undiscounted new phone).
They may "mean business" but frankly, they suck at it.