Somewhere we decided to accept crap system and device security as normal because oh we'll just firewall it. That was never a good idea but the more cloud connected things we deploy it becomes completely untenable.
Large corporate networks are already hostile territory due to BYOD. The only way to maintain the firewall as anything other than security theater is to lock everything down so much that nobody can get anything done.
The whole approach is braindead. We don't see how stupid it is because it's grandfathered in.
EDIT: My simple solution (the one I had in mind) is to make a firewall box - a cheap linksys router with custom firmware can be set up to act as a switch but will also follow iptables rules. This serves the same sort of purpose as a hardware firewall in the middle of a corporate network but at lower performance. Bulbs connect to switch/WAP outside of firewall, hosts connect inside it. Everything behind it is then restricted from accessing the bulbs or viceversa.
Without firewall: Here's direct access to everything on my network.
With firewall: Oh no, you can hack all my smart lightbulbs and change their colours.
This kind of thing is exactly you need firewalls for. Without a firewall this could pose a serious threat, with a firewall it's probably a good practical joke at best.