aw, poor shrimpy :(
Yeah, it really depends on the betta. My husband has a ghost shrimp (he got 2, one died, he refuses to buy another). His betta, Sushi, completely ignored it. In fact, I noticed the silly shrimp would swim up and try to nip at him! I was taking a video and actually caught the little brat on camera grabbing and yanking Sushi's tail! Still the betta ignored him.
Sushi died last month and a week later my husband got Shisu. He definitely doesn't ignore the shrimp. Not to say the shrimp probably doesn't deserve it... hehe
But he doesn't get too bad either. What I see happening is that either Will (shrimp) will head butt Shisu, and Shisu will turn around and head butt him right back, or Will just gets too close to Shisu and the betta either swims quickly away (wanting to avoid the shrimp's claws) or does a quick turn and swats him with his tail or head.
His behavior towards the shrimp is entirely a reaction to the shrimp being a brat to him first, rather than seeing him as lunch.
I think all tank mates are like that. MY betta has an african dwarf frog in his tank. He was absolutely fine with her... and I got a 2nd female african dwarf frog... and that one had a bad attitude problem too. I thought my betta was being mean to her, until I realized that she instigated it. I found her a new home with an employee at petsmart. It's been a couple of weeks, and I saw the girl who took her a few days ago. She said the adf is in a nice sized tank with only snails, and she is just as bold and aggressive towards the snails (she seemed surprised, like she didnt believe me that the frog was antagonizing the betta)
With tank mates, it's a hit or miss. Just like people... we generally can live in a town with other people, but who's to say you are going to get along with your neighbor?