Paperwork Reduction Act is an attempt to reduce paperwork. Customs and immigration questions are not paperwork, despite how systematic they are. Consider if they have an OMB number for "Citizenship? Duration of stay? Purpose of visit?", the questions nearly every traveller gets asked. They don't. Of course they don't. They don't need one.
PAPERWORK REDUCTION ACT STATEMENT: An agency may not conduct or sponsor an information collection and a person is not required to respond to this information unless it displays a current valid OMB control number.
These questions are present on customs form 6059B. Asking them again verbally probably isn't illegal. This form does have an OMB number: it's 1651-0009.
