They use the fact that Shari'a court exist in some country of Europe, or can be theoretically use in some case in the US, mostly around marriage and divorce, as a boogeyman for the Shari'a slowly taking over the West.
It's really crazy when you start pondering on the giant difference between what really exist (boring offices who deal with couple issues all day) and the fantasy of those "anti-sharia" group.
It's potentially very dangerous because you cannot be muslim without believing in the shari'a law, since it's literally the path which lead to God in the muslim faith, so there is a very big dissonance between what the word mean for western general public (cutting hand, stoning..) and what it mean for everyday muslims (praying 5 times a day, don't lie, respect the rights of your neighbours..).
The jewish community have the exact same court (halakha court), to deal with the exact same issues. Certainly, 50 years ago, those same people marching against shari'a would be marching against jews taking over the West.