Because they could be swallowed by the animals and make them sick.
Omsin, a 25-year-old green sea turtle, was resident of a small public park in Chonburi Province in Thailand.
Visitors to the park would frequently throw coins into the pond, because tradition holds that sharing one's wealth can make the coin thrower live a long life, just like turtles normally do.
Unfortunately, that's bad news for the turtles.
After years of swallowing items thrown into its pool, it had become difficult for Omsin to swim.
When the turtle was discovered with a cracked shell, conservationists sent Omsin (Thai for "Piggy Bank") to veterinary scientist Nantarika Chansue of Chulalongkorn University, who is Thailand's leading turtle rescuer.
A CT scan revealed that a pile of slimy black coins in Omsin's stomach was limiting its ability to swim.
A gruelling seven-hour surgery followed on Monday, which removed a 5kg hoard of 915 coins in various currencies.
Omsin is okay now, Dr Chansue told Agence France-Presse, but will have to remain at the centre for up to two weeks.
Vets have been trying to curb the practice of throwing coins into turtle ponds. "Throwing the coins is more bad karma because it is animal torture," said Dr Rungroj Thanawongnuvej, dean of the faculty of veterinary science at Chulalongkorn.