I don't believe that's quite accurate. I've only seen the video of the most popular front angle, but it seems it landed in her row near her feet on the ground, between her and the guy standing besides her. She and the guy beside her reached down, the dad also came over and reached over the seats and picked it up. We can't see the hands or the ball (from that front angle) but she was probably upset because it was near her feet so she thought it was "hers".
I'll give him the benefit of the doubt since he seemed like a cool guy and was willing to let her have it because he didn't want any drama.
I have a similar story. When I was small, around 5 years old, I was standing on the side of the street with my family watching the local Santa Claus Parade. People on the floats were throwing candy into the crowd. A full-size Oh Henry bar landed literally right at my feet. I reached down to get it and some teenager snatched it before I grabbed it. I was disappointed and that's a bit low to take from a little kid, but I guess it was fair game. My mom or I didn't yell at him lol. Much different though than an adult snatching a ball in front of another adult, which is 100% fair game.