You sure do hold Christians up to a higher standard.
I hold them only to the standard they claim for themselves.
But I do agree that Christians should not support slavery and I don't see the connection - sorry.
The connection is that Christian Dominionists want
their scripture, ie, the 10 Commandments posted for
all to observe & honor. It's troubling to use schools
run by government to proselytize to non-believers.
It's heavy handed, & un-constitutional. Moreover,
Christianity has a checkered history that's unworthy
of following. The 10 commandments are variously,
irrelevant, good, evil, incomplete, & even ignored
by the very Christians who trumpet them..