Your point, I presume, is that: (1) The meaning of a monument includes, at least in part, the intentions of those who caused it to be erected. 

But it is also true, I suggest, that: (2) The meaning of a monument includes, at least in part, the response of those decades later who live with the monument, regardless why erected. 

The first "principle" would seem to justify the 40 foot memorial cross as not promoting religion, but the second principle does not, for clearly some atheists profess offence. Thus, controversy is reasonable, for principle are in conflict. 

The confederate monuments erected as assertions of white supremacy and black intimidation would appear to fail on both principles. Thus, arguably, controversy is not quite so reasonable.