This is one of the weirdest (I'll refrain myself from using a stronger word) reason for merchandise drop and it goes against logic. My team is better because of him, but fans are not satisfied and pay less...what??
The nationality thing certainly affects the merchandise sales, and I think many feel the effect is too strong. However, I guess it could be argued in many ways. For example, you have the (home town) fans who hope your team does well. They may be excited about the new star and after a few games, start buying more merchandise. But perhaps your team has wide national appeal due to your domestic players. They don't care about your team and its success, they care about the players. A new foreign guy who takes minutes from the local players may not be welcome to those fans, and you see a decline in sales.