I think it may have been a case of, one employee was cleaning tanks and was in the process of finishing the corn snake's tank but had to leave (shift ended, sudden emergency in the store, etc) and did not get a chance to tell the next person on shift. Who knows, BUT she did sound like she was concerned and promptly fixed the error. With the ball pythons, while I don't agree with that practice, I can understand why so many would be in the same tank. These ball pythons are not meant to stay there for weeks and months. They are meant to be in for a few days at most and sold. Though I do think they should have hides and temperatures need to be checked several times a day.
If you want to see change in that store, go above them. Get in touch with someone higher than the store manager (district manager or even the top CEO if you can) and let them know about the conditions in the store. Let them know they will be losing you as a customer and you will be going to another local store for your pet needs and will be bringing several of your friends and family along with you. Tell them if they'd look into it and conditions in the store improve, then you will come back as a customer. I know that when I worked in retail, if someone went above us with a complaint you bet the store would have it fixed soon.