According to the link it's because they are replacing the trolleys, not putting them back isn't mentioned.As the trolleys are about 16 years old and made to last 25 years seems more cosmetic then anything.
I have a special disc, from Hearing Dogs for Deaf People, which is on a keyring. It works instead of a £1 coin in trolleys.Saves having to find the coin. Not likely to forget to reclaim it. It's got my cars keys on it!