Why isn’t it considered borer? your are rinsing off the fat or dirt that is stuck on?
siman 323 sief 6 says its permissable. why
One may wash dishes on שבת, even though the water separates the dirt from the plate. This is because the regular maintenance of an object is not considered selecting but rather an act of cleaning. Dishes normally get dirty and are washed, so washing them is permitted.
See Shmiras Shabbos Khilchoso chap. 3 ftnt. 7 that brings this idea from R’ S. Z. Auerbach zt”l