Dear Apple, please use industry standards. BACnet is a ISO, ASHRAE and ANSI protocol.
As I understand it, BACnet has a number of drawbacks that make it unsuitable for this kind of thing. In particular, BACnet is not really plug-and-play (and is not designed to be), supports a relatively small number of transport protocols (one could imagine this Apple system eventually encompassing BlueTooth, ZigBee etc). Furthermore, it's not really a secure standard as I understand it - access to the WiFi network would essentially give control of all the devices. Current solutions aren't really much better, but it needs to be the primary concern (and I suspect that Apple have considered it). As one example, BACnet best practice is currently to have parts of the network that have to be in userspace (temperature sensors, door sensors (as opposed to lifts, aircon)) is to place them on separate physical networks.
Even moreso, it's kinda usually implemented right above the link layer (which violates the hourglass principle, if nothing else), and there are issues that will come up in the near future with the alternatives (BACnet/IPv6 being an issue (i.e. most BACnet/IP devices will stop working as networks switch to IPv6)).
BACnet is probably fine in the enterprise domain, where everything is installed at building construction time, there are people who manage this stuff for a living, and things like airgaps are desirable as well as necessary. In the home domain, it's at least overkill, and makes things incredibly difficult for users. It's entirely fair for Apple to go their own way.
Trying to do a standards-based approach at the moment would be a horrendous idea - firstly I know a large number of companies to be trying to come up with their own solution at the moment, but secondly it would just end up being a standard by committee (with no real implementers). Let the competitors hash it out for a few years, and then standardise the best one.