The RSPCA are targeting these people because its more sensational in the media, I agree that being old, disabled or a child is not a get out of jail free card, and if they are causing suffering or cruelty they SHOULD be prosecuted, if you take a look at the cases that are refered to, in every case, there was no real reason to mount a prosecution on the grounds of cruelty or neglect.
So why would the RSPCA do this ? PUBLICITY.
The RSPCA have a conflict of interest when it comes to prosecutions, they get loads of publicity and make loads of money from donations, the more severe the case is portrayed the more donations, hence pensioners, disabled, children etc will generate more revenue.

Oh, and have you seen the sexual harrasement and bullying case the RSPCA have just spent £30K setteling with an ex Inspector ? take a look at
Time for a review of Animal Charity's Bullyboy Tactics ?