Precisely. There's no guarantee you'll actually get that "free mini." Not to mention, it's not a free model - it's one you've already paid for. Plus now you have to ship it back to them, so there's postage involved (assumably, I'm never returned a mini to GW myself). You have to wait however long it takes for them to send you a new one. And, of course, the cherry on top is there is no guarantee that what they send you back isn't as bad or worse that what you had replaced. It is just one gigantic, wet, sticky furball of hassle - and that's if everything goes smoothly.
Now, to be sure, I've had problems with other resin minis from various producers in the past. Still, not to this degree.