I'm going to guess that apple's products warn about the danger in using non-apple products - risks of damage and all that. Not wanting to ruin your new, expensive phone, you play it safe and just buy the matching charger, even though you feel like you should have been able to use the phone with everything that was in the box. For some years, Apple has warned about the dangers of third party accessories possibly ruining your stuff, after all. And I'm guessing the apple store employees aren't exactly going to recommend you buy other brand's stuff.