Constant tearing of Levi's 501s

My husband has been a loyal fan of Levi's jeans for many years since the company is typically known for quality and good fit. As such, I bought him four pairs of the 501s in order to refresh his wardrobe but was very distressed to see how quickly they tear in the crotch area. The first pair lasted a month and I was able to return it for a new pair quite easily. The replaced pair lasted for two months and I had to fight with the store to replace the jeans when they tore again. I was told that I was not supposed to wash the jeans and that was why they tore. I was given a replacement in the end, but my original receipt was taken away. Since then, two more pairs of jeans have torn in the crotch area without any warning - my husband has literally stood up from the table and they have torn. The jeans are properly sized and well cared for. I would estimate that I haven't even washed them ten times. This is incredibly disappointing since these jeans are very expensive.