ZigBee is an open wireless standard, designed to support lightweight communications between devices with small, low power radios. Internet of Things devices primarily.
Think of it as a lightweight analog to WiFi or Bluetooth.
Now imagine if your WiFi router manufacturer decided to lock out all devices that hadn't been through its own approval process. Or your Bluetooth devices decided not to talk to other Bluetooth devices from different manufacturers.
It would be chaos. This is the exact reason why these sorts of standards are developed, to provide the consumer with a consistent experience that doesn't lock them to a particular manufacturer.
Philips thinks differently about this though.
Its Hue lighting system will now no longer work with other lightbulbs supporting the ZigBee standard. Presumably because that might introduce some competition on price. Philips will certify products from third parties, however there will inevitably be a cost and delay in achieving this.
Philips claims that the move to block third party bulbs is because they can't guarantee that these will work with their system updates. Nonsense, so long as Philips sticks to the standards there should be no issues. It's a light bulb, not a nuclear reactor.
What Philips should have said was "Although our system is based on open standards, allowing other manufacturers products to work with ours forces us to compete on price and quality, both of which risk damage to our bottom line."