I think there are a lot of facts here to be taken into account. Also there is misinformation, but not coming from anyone on here I may add.
Expense: There is a general belief that rodent pets are cheap and easy to care for. In some ways this is true, though keeping rodents properly, especially piggies I feel, requires more time and effort that many people realise at first. As for being 'cheap', yes in theory, once the original set-up is in place and you have toys and enrichment, you buy veggies, bedding and hay and pellets. Not the most expensive pets in the world. Plus there is the perception that guinea pigs are hardy little animals.
Now I already knew how expensive a trip to the vet can be from my other rodent pets, however it's not too hard to see how suddenly being faced with £100's vets bills can come as a nasty shock. I took one of mine to an out of hours vet once. The consultation alone was over £90. A scan would have cost, if I recall, over £200, anything else was in the £100s. Fortunately for my bank account she didn't need to be taken in as an emergency (as I originally thought she might). She had colitis after eating grass and was bleeding out of her bottom. I didn't dare leave it until the morning, but had it been serious can you imagine the bill? £1000 perhaps?
It's easy to berate people over not wanting to pay vets fees, and easy to say "well don't get a pet then" but even I was shocked at the cost of out of hours care, and I DO have money set a side for these eventualities.
I actually think that it would be more helpful if it was a general policy of pet shops to let prospective owners know that vets bills can be large. Of course they won't do that, as it may put some people off from buying the pet, but they should be informed before purchase to save a lot of heartache and suffering. They should also be informed of insurance plans and other help, before purchase.
Which leads me on to Being Informed - Research:
Not enough people totally understand what keeping their pet will involve and discover that things are not as they imagined them to be. Fortunately this forum is a great mine of information, and there are other forums out there for pigs and just about any pet you can think of.
Fortunately many people really do understand they have a duty to care, and even when their pet's behaviour or something else has taken them by surprised will seek out help to make things right or as best as it can be. People have misconceptions from early childhood attitudes of piggy-keeping. Things have changed. It can take a while for people to catch up!
It's great to see more people doing their research first, but there are plenty who already believe that they know the necessary information via their childhood experiences, or from what pet shops have told them, and it doesn't occur to everyone to seek out forums first-hand. Times are changing. For younger people the internet can be the first place they think of for advice, I think the old misconceptions will eventually die out.
And here we have the duty of care.
In an ideal world people will have taken expense, including vets bills of potentially £100s, into account before taking in their new pets. They will have done their research and pretty much know what to expect in terms of animal needs and behaviour.
In reality people can be shocked at what caring for their new pet entails, especially the expense, and also shocked at how frequently health problems can occur in an animal with a hardy reputation, but duty of care extends to doing the best that you can in the face of these surprises to make things right, and if you cannot give appropriate care to your pet then rehoming to someone who can, often best done via a shelter.
This is why, if I had my way, it would actually be enshrined in law for anyone selling pets as a business to give out approved (perhaps by the RSPCA) guidelines on the care AND potential expense. Not something wishy-washy like 'you will agree to take the pet to the vet', but something more solid, that your pet is likely to require vet treatment at some time during it's life and that may include scans, x-rays, sedation, operations, tests, and ultimately £100s in treatment.