As a wifi engineer for my day job, I have some ideas on why they did it the way they did. I don't work for Nikon though, so these are just my personal theories.
Overall, my expectation is that Nikon went for simplicity. By having the camera be effectively the wireless Access Point, the only other device they have to worry about interoperability with is the smartphone/mobile device. If they were to connect to an infrastructure wireless system, they would need to also ensure interoperability with the infrastructure manufacturers. While the WiFi Alliance is an industry organization that certifies interoperability across wireless vendors, there can still be caveats that come up.
Additionally, most wireless networks are not simple "connect and use" types of networks. Most stadium environments (for sporting venues) are very similar to the experience you get when you connect to the wireless at Starbucks. Your computer/mobile will connect fine, but you can't do anything until after you launch a web browser and at a minimum accept a terms and conditions page, or worse you have to provide some information like an email address. To do this with the camera, they would need to also embed a web browser, and potentially a more user friendly way for entering text like an email address.
My Nikon D5300 by default provides an open (unencrypted) SSID, so the dedicated wireless network is not about securing the media itself, as much as for simplicity. There is an optional setting for enabling WPS security, however that is also trivial to bypass and focused on providing a simple user experience.
Now, in a stadium type of environment (like the NBA example referenced), the way they implemented the wireless functionality is actually the worst way they could have done it. Since wifi uses unlicensed spectrum, it's all shared. If we had 5 photographers plus the goal cam all on the same wireless network, we could give priority to the cameras over the fans in the stands who are using the same wireless network to share pictures to Facebook or videos to Youtube. However, with each camera on it's own wireless network, but using the same frequency ranges, wireless is more likely to be congested and unpredictable. My expectation is that professional photogs shooting a sporting environment like an NBA game, if they shoot tethered, are most likely doing so wired rather than wireless for this reason.