Unfortunately this is an all to common occurrence. The #1 place I used to see this was at Walmart. If you come upon a store treating there bettas like this please snap a picture and then look for the contact info on that stores website page and let them know what is happening. Dead bettas should not be seen in any stores if they are caring for them correctly. If you have different stores in your area with this problem please contact me with the info and I will add it to this list.
Walmart
http://corporate.walmart.com/contact-us/
Petsmart
http://www.petsmart.com/helpdeskcategory/021414-us-helpdesk-about-pet-smart-category-cmp_20140214_223430?article=cmp_20121115_172158
Petco
http://www.petco.com/petco_Page_PC_contactus.aspx
Walmart
http://corporate.walmart.com/contact-us/
Petsmart
http://www.petsmart.com/helpdeskcategory/021414-us-helpdesk-about-pet-smart-category-cmp_20140214_223430?article=cmp_20121115_172158
Petco
http://www.petco.com/petco_Page_PC_contactus.aspx