Why are ball pit balls aged 3-4 and up?

Hi,
We own this bouncing thing: http://www.amazon.com/Intex-Jump-Lene-Transparent-Bounce/dp/B002AJ8S9O/ref=pd_sim_t_2
Last night we went to ToysRUs so get some balls for it and I inspected them in the car and notices they were not sealed and had dirt, mud and even a leaf on them so we will be exchanging them tonight but I also noticed it said it was for ages 4 and up… My kids are one and two. I can’t think of why they can’t play with them other then the fact that those ones looked like they had been returned by someone. They’re too big to choke on and have no pieces. What could happen? I’m also not going to be leaving them alone with them.
I looked it up but I can’t find reason for it.

Thank you.

Ok this is funny. It says “Cherish this safe, cozy and interactive place for baby to hang out and have fun.
Safety warning: This product contains small parts that may present a choking hazard for young children.”

http://kids.discovery.com/shop/baby/discovery-dinosaur-ball-pit

Not sure why either, my LO loved playing in them around 2 years old, never had a problem with them

We went back and found all except one had been opened. They all had dirt and one had a wad of hair in it! They sent them all back as soon as we showed them. We bought the one that was sealed and I just got done washing them. I still don’t see what could go wrong for them.