He wasn't "let off by RSPCA". They cautioned him.
What has to be borne in mind is that the RSPCA, as a charitable body, can only bring private prosecutions, and those cost money. Such prosecutions are obviously expensive and are not funded through the public purse.
It would be akin to one of us, say, suing another individual over, lets say, a boundary dispute. Fine if you can afford to do it, but virtually impossible if you can't.
We only have to see the results of the more serious prosecutions brought by the RSPCA reported on the news from time to time. How often are we all left scratching our heads in frustration at the lenient punishments meted out to offenders?
I share the anger of every true animal lover but unless the fincancial situation of admirable organisations like the RSPCA improves significantly, matters will not improve and animals will continue to suffer, even die, in the wrong hands. Like it or not it's a question of priorities governed by insufficient funding.