A cat/some cats shit in your gardena and you therefore think that people who cause deliberate cats of to cruelty to innocent animals are brave 
A Ferrit almost killed me once. I don't have an overwhelming urge to dump them all into wheelie bins. I'd hope that I'd seek metal help if I did.
Birds shit on my windows all the time. There is one particular Seagull who I am convinced takes pleasure in shitting on them 30 seconds after I have just washed them. As yet I have not had any urges to put him/her in a wheelie bin.
And OP YABU. You say it's not as bad as strangling a cat, but if the cat had not been found it would have faced a far more painful, slower death than strangulation.
When Mary Bale put the cat in the bin she had no idea when the bin men were due or when/if the cat would be found.
She says she doesn't know why she did it and it was just a spur of the moment thing. Then why she did not go to back to the house, when she realised what she had done to check that the cat had been found safely? Oh, yeah, that's right. It's because she didn't give a shit. She could do this sort of thing all the time for all we know, but just ahsn't been caught until now.
I also agree with BFF, not punishing her would have been giving people liek LongStocking the green light to go ahead and copy her.