Southeastern Guide Dogs requests that puppy raisers only give their guide dogs in training approved toys. These include Nylabones and Kongs. These approved toys seem to be the least destructible, thus the least dangerous, although no toy is indestructible. The black Kongs are actually more durable than the red Kongs. Some approved toys can be ingested so supervise your pup closely. SEGD asks that puppy raisers do not give the puppy any toy that he/she can chew off bits & pieces of and ingest. This includes; rawhides, pig ears or hooves, soft sheepskin type toys, plastic or stuffed toys, toys with squeakers or sounds, empty plastic bottles, Frisbees, rope toys, and no balls, which means no TENNIS BALLS). According to page 31 of the SEGD Puppy Manual; 2008 edition, all of these "no" toys can be ingested, are dangerous, have chemicals or are inappropriate for advanced guide dog training.
Tennis balls seem to have many uses outside the tennis courts. They may be on the bottom of chair legs to prevent scratching a floor or placed on the end supports of a walker to help it glide more smoothly. It would not be good if a guide dog, veteran's assistance dog, or ambassador/therapy dog lunged towards one of these tennis balls. It could cause an injury to the dog handler or the person using a tennis ball for another good reason besides hitting a winning shot over a net.
A wonderful blog post written about approved/non-approved toys by a fellow SEGD puppy raiser was posted recently. Thanks to CD McClean for sharing this link about her puppy Jam and his "Duck Buddy".
ps....the cute pup pictured with the 3 tennis balls in his mouth is a model, not a SEGD puppy in training.