Arguably, the same reason judges should be allowed to remove jurors who admit their intent to convict an innocent defendant: jurors must apply the law impartially. The LII has a pretty decent primer on all this entails[1].
And almost all state laws require a jury to unanimously vote to convict, so empaneling a jury that has someone who will always acquit means that the jury's decision has been set from the beginning.
Personally, I live in a state that requires a unanimous vote to convict or acquit, and prohibits the judge from "poking at" a deadlocked jury[2]. In the few times I've been selected for a jury I haven't gone far enough to be asked if I would be impartial, and I'm glad because I've yet to decide where my personal ethics take me.
[1]: https://www.law.cornell.edu/constitution-conan/amendment-6/i...