In Zobrest, they used Witters and Mueller to argue that it was legal to for a federal program to assign sign-language interpreters to religious schools to help the deaf.

In Mueller, they found it okay with tax deductions because it was for all students. They, the government, won't exclude people. Also it was the private parents choice, not the government.

In Witters, they used the same process as Mueller to get to the answer, that helped students go to religious schools, who wanted to be pastors.

These three cases are neutral towards religion is why there was no challenge under the Establishment Clause

They were choices of the private individual, then there is no case.