The absolutism of this assertion is what makes it incorrect. Period.
I realize a lot of people would like it to be absolute, but it just isn't. In those cases where accepting the 2nd assignment is prohibited, the reason for the prohibition is not because there was a prior assignment but because there was an improper contingency for the 2nd assignment. That improper contingency can be established with or without the 1st appraisal, so that right there tells you the variable is not the fact that there was a prior appraisal.
It's too bad if you don't like it. It is what it is. If I received a "comp check" request that didn't have those strings attached, I absolutely could render an appraisal in full compliance with USPAP, and depending on what the client's criteria are for moving forward I absolutely could render the second appraisal in full compliance with USPAP. That is to say, that if I came across new information that was not available during my prior assignment and it affected my value conclusion I wouldn't have any problem coming up with the different conclusion.
I daresay there are a lot of appraisers, including you, who would do the same if faced with two assignments on the same property each having a different SOW. The fact that it can happen is all the reason we need to disprove the absolute assertion that it never happens.
And while we're at it, I can also review a sales contract without allowing that knowledge to contaminate my appraisal, and I'm sure I'm not the only appraiser who can do it.
Where you folks are going wrong is in trying to read things into these sections of USPAP that aren't there.