Yes, I've also heard they are dangerous for dogs. The real tennis balls are made with a certain kind of material that is dangerous. However, we have tennis courts right over our backyard fence so sometimes we (or the dogs) will find a tennis ball outside. Luckily they do not chew them to pieces, so I'm not worried about it.
They do sell "tennis balls" that are specifically designed for dogs to chew on. Is that what you got for Ryder?