All Petcos are 'different'.
First of all, the price for those corns are extremely high. My local Petcos sell the all the corns (striped, amel, snow, anery, normals, etc.) for $39.99.
The Petcos and PetSmarts are generally geared for 'traditional pets' (cats, dogs and fish) and do not really specialize in reptiles. I've purchased some healthy New Caledonian Crested Geckos at my local Petcos and PetSmarts. I've seen some very nice healthy looking Corn Snakes at my local stores.
I've read a lot of 'horror stories' throughout the various reptile forum sites I am a member and I believe that most stories are true. In my opinion and observations and like the previous post from 'Dionythicus', all Petcos and PetSmarts are different. It all depends on who is assigned in taking care of the reptiles and how knowledgeable they are in herps. I've been to Petco stores where the staff take very good care of the reptiles very well and I've been to some that don't. The ones that have knowledgeable staff and keep their enclosures clean and sterilize empty enclosures before putting a different reptile from the previous one in it, are the ones I fequent, only because I know that the animals and reptiles are well taken care of.
There was one Petco store that I went to recently that had very nice baby Corn Snakes. I asked one on the young staff members if I could check them out. When he opened up their tank, he appeared hesitant and scared. One baby corn snake was a little feisty and attempted to strike at the young staff member. It was well understandable, since baby snakes can be a little fussy. The young staff member started to throw aspen at it and said, "how do you like that?!" as he kept on throwing aspen shavings at the poor cute little corn snake. Needless to say, I got upset and was disturbed by his actions that I told him to stop and leave the baby corn alone. I told him the snake was startled by his quick movements and the way he was handled and to just leave it alone! It's not going to help any future customers want to get any of their corn snakes if he continues to agitate the snake. After talking with him a little more, he did not know much about the reptiles that they have. I suggested that if he is going to continue to work at Petco, that he do some more research on the animals and reptiles, so that he can be more helpful to potential buyers. After talking with him, I talked with the manager about one of her young employees and what he did to the snake. She said that she will talk with him. If I was a manager at a Petco or PetSmart, I would hire someone that has adequate knowledge and experience with the animals.
If you see a dead animal/reptile or notice something wrong with them (like mites or lack of humidity) in one of the enclosures, then address your concerns to the store manager. They usually take care of the issue right away. If they ignore the concerns and are obviously negligent, then report it to the proper authorities.
As for mites, it can be a pain in trying to get rid of them, but it is treatable (sometimes, its not as fast as we want it to be).