Like people on here, some people might be on benefits and will go without themselves to make sure their cats are well looked after, and no-one could say that they're bad owners or they don't deserve to own pets simply because they're not working for whatever reason. Personally I was looking at cats when I was on Jobseekers but wouldn't let myself get one because I wasn't in the financial position to (had myself and OH to support from one claim = no money to spare!) so I decided to wait until I had a job and had a bit more wiggle room with our finances. If you have enough money to provide adequate care for the cat, then it doesn't really matter where the money comes from.
If you're on benefits, adopt a cat even though you don't really have enough spare money, and then expect charities like the PDSA to pay for everything like chipping, neutering etc then that's a completely different matter and I think that that's wrong.
ETA - I've just read the thread on MSE that you're referring to Fire Fox. I can't help but say that it annoyed me, the lady has just adopted the cat and she already thinks she's entitled to a whole bunch of freebies as she's on housing benefit. I try not to rant too much about things like this, but it's just taking the mick and abusing what the charity is there for. If she can't afford possible vets bills, then she shouldn't have got the cat in the first place. Fair enough if a person's circumstances have changed and they find themselves needing to use the PDSA, that's what they're there for, not as a 'get out of paying' free card!