There is nothing inherently wrong about tightening immigration policy. I don't know the details but there are couple of perfectly understandable reasons why people oppose immigration (note that they overlap a bit):

1. Immigrants are disproportionally responsible for criminality.

2. Switzerland is a rich country and the Swiss want to keep the wealth for themselves. But of course, many immigrants have a net positive effect on the country's wealth.

3. Many immigrants come from a very different culture and don't want to adjust to the mainstream culture.

4. Immigrants may form groups isolated from the rest of the society.

5. The Swiss culture is different than the American one. They're not that welcoming to immigrants. They want to keep their culture. I personally understand why they voted not to allow mosques in their towns.