Here have been several post likes this, including me!
Have you asked them yet, what are their reasons for saying no?
Benefits of having pigs inside:
More interaction, piggies will be happier.
In for the warmth, they do not cope well with cold weather.
Less likely to get bored of them.
Easier to spot illness.
Maybe you could offer to help pay for the indoor cage? A c&c is best. Many pet store cages are not suitable and piggies like the extra space. How piggies have you got?