If I keep my cats inside?

...but now, I agreed to adopt 2 cats from a local shelter - they want me keep cats inside. 

I asked my vet (Moko's vet) I was thinking to have one or 2 kittens, so if he had any idea. Then he gave me a phone number, it's a lady's from the local cat's shelter. (These shelters don't allow you to adopt cats/dogs if you were single, so I hesitated to contact them...)

The lady from the shelter asked me if I could take cats, although they were not kittens. (They were not in the shelter, too) They were stray cats, the guys from animal shelter caught them  for spay surgeries. She said if I didn't take them, she had to let them go - and she was worried that they might not thorough the winter.  

She said she let me borrow her cage for the cats, means - they are so wild enough I need a cage to keep them my home... ? Of course I won't let them out, they would go straight back home once they get free. I have to keep them inside whatever my thought is - actually, I still think it would be nice for cats being free, run field, hunt little creatures, have fights sometimes - even it makes their life short. 

I'll keep them inside, anyway. I just hope they'll get used to the new home soon.  (I've not seem them yet, so I have no idea if they are big and strong or not. I don't now when they come to me, too. Maybe soon. )