It's actually the other way around - the requirements of USPAP include adherence to the law. In the event of a conflict the USPAP requirement that conflicts is always subordinate to the law. That's what the JURISDICTONAL EXCEPTION RULE is for - to retain all the other requirements for the assignment + adherence to the law where the one or more sections of USPAP conflicts.
Here's an example: lets say a state or federal law or regulation or judicial order prohibited the analysis, consideration and disclosure of the subject's prior sale or listing history in an appraisal. USPAP SR1-5 normally requires the analysis and SR2-2.x.3 normally requires the disclosure of such activity within 3 years of the effective date - that conflicts with the established law.
The appraiser is not stuck between a rock (the law, rule, regulation) and a hard place (USPAP requirements) because the JE applies and gives the appraiser an automatic exit from the perceived conflict. The appraiser has no discretion on the matter, so it can't even be considered discretionary or an opt-in on their part. They are REQUIRED to adhere to the law and to not develop or report that sales history analysis for those assignments so affected by that law. We don't "choose" to comply with the law instead of SR1-5, we are required to do so.
The appraiser's solution is to comply with the law and refrain from analyzing or disclosing the prior sale in the appraisal and appraisal report in tat assignment. The JE severs the USPAP SR1-5 and 2-2.x.3 requirements for that assignment without touching any of the other requirements in USPAP, the end result being an appraisal report which meets all the *applicable* requirements for that assignment in compliance with both the law and USPAP.
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Now to your point, appraisers can always CHOOSE to adhere to USPAP in an appraisal assignment, but under normal circumstances the applicability of USPAP is otherwise normally triggered by a requirement in either the law and/or by agreement with the client or users. So if govt deems certain "value opinion rendered on an impartial basis" assignments to be Not-an-Appraisal then USPAP would not be applicable to those assignments insofar as far as the govt is concerned. That wouldn't necessarily prevent an appraiser from opting into the various requirements of USPAP anyway, but they wouldn't be required to do so.
Another way would be if the user drove compliance of certain elements of USPAP which were not already included in the requirements for an Eval.