Remember - guests have been locked out of their accommodations in the middle of the night because of this. Airbnb has cultivated an ecosystem based on iCal syncing. How can you possibly say that Airbnb bears no responsibility for this? It is on Airbnb because they have for years now provided information that they know is used downstream and then took it away without notifying anyone. And it has affected hosts and guests alike. There is no reasonable explanation for this approach if it is in fact deliberate on Airbnb's part.
Next - let's look at the actual security implications here. It is not possible to simply 'scrape all the iCal files from Airbnb' because each iCal file has a token associated with it - without the token, you cannot get the iCal. There are issues with this - for example, you cannot reset the token, so once you give out the iCal to someone else, they can pull the iCal forever. Some PMS platforms like AirGMS allow you to reset the token and invalidate previous iCal links - Airbnb should offer this functionality. Perhaps the token generation mechanism was compromised and this became a zero-day exploit. But, if so, Airbnb should have been all over the communications on this one. So, while it is not the most secure mechanism, it is also not the most insecure mechanism and the threat is not nearly as big as you indicate.
Raising this as a security issue is perfectly valid and, if Airbnb has decided to pull this information, then it is their responsibility to ensure an orderly transition to a more secure solution by openly communicating with the community the reasons for doing this, the timeline for doing this, and the alternative mechanism they are providing to ensure downstream dependencies are not broken.
But putting all that aside - Airbnb has been pushing a move to a more professional platform for a while. So, while moving reservation data to a more secure interface, they are now stiffing the exact type of users they are trying to grow - sophisticated, multi-listing users that use technology to ensure the best guest experience. And these types of issues create openings for HomeAway & Booking to exploit from a competitive standpoint. So strategically this approach makes no sense, so my money is still on 'this is a bug'.