Because public WiFi has an ongoing maintance cost, plus the cost to set it up and properly isolate it from your network. The magic box is just easier. You aren't responsible for it. The magic box (via LTE) also has vastly superior QoS so that one person won't bog it down.
My parents house is one place where a magic box works amazingly. Their only internet option is 3 mbps DSL. B26 only on phones, and that's upstairs only. The magic box latches on to b25 and provides 15-30 Mbps consistently. They actually use it now with a Sprint mobile broadband plan. For some people, the magic box is a better solution than an Airave or wifi calling (which won't work well on 3 mbps DSL if someone is using the internet). For businesses, it's a $0 cost, easy deployment to help customers. And they aren't responsible for what people do on it.
I really don't see why it is that you think that if you don't have public wifi then you wouldn't use an MB. The MagicBox is a repeater for Sprint's network requiring essential zero exposure or expenditure for the retail provider beyond electricity. It is also zero maintenance or setup. It's a highly superior solution in my view.