From what I've read, the preK classes are funded by federal Chapter One money, which comes with strings attached. The money, I believe, is meant to boost school achivement among disadvantaged populations. St. Mary's used to offer preK at the schools with the most students receiving federal lunch money, and I believe they still do this.
But this is the first time I've heard of income limits being used for admission. The classes used to have a screening test to target the children who needed preK the most, such as kids with learning disabilities.