This is not what we want here ;). We are sure that these params exist. We want to know if the user has overriden them. For example, if you pass a NiftiMultiMasker, whose smoothing_fwhm is custom, to CanICA, the custom parameter will be overwritten by the one given to CanICA. That is why we want to know is the value of the parameter is None (default) and if it is not, that means that the user has put custom value, and then we warn him that we override it.