I can see that Garmin are trying to pass this one off onto their distributors and retailers, but we hold our stocks of electronics for days or weeks and not months.
If Garmin did not ask their distributors (like SMG) to replace the instructions in their existing stocks then I don't see that failure to make good on promised functionality can be anyone but Garmin's responsibility.
The other problem is that someone thinking of making a purchase might download a manual to see what they are getting for their money, at the time of preparing to make said purchase the information published by any manufacturer is going to be "of the day". If they then decide to pull features from the product they will have to answer to anyone who made a decision based on that.
Of course the product in this case still has the function available, it just wont work as elegantly as promised. I have in mind a car that is advertised as having an automatic breakdown recovery service, which later turns out to come with a mobile phone included.