State courts decide federal constitutional questions every day. Those decisions may ultimately be subject to review in federal court, but, as an initial matter many federal constitutional questions are heard in state courts.
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I suppose they can do whatever they want, but their decision isn’t worth the paper its written on.
It's not what they "want to," it's what the Constitution commands:
"This Constitution, and the laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land; and the judges in every state shall be bound thereby, anything in the Constitution or laws of any State to the contrary notwithstanding." (U.S. Const., Art. VI).
Literally every day that courts are in session, state courts decide whether or not state laws are constitutional under the federal Constitution; and their decisions stand unless and until the U.S. Supreme Court reverses them. So they are often worth much more than the paper they are printed on.