Another possible interpretation of "to avoid becoming tainted" is that it refers to avoiding "residual knowledge" that in some circumstances can be used to make a copyright claim. E.g. if someone reads the ZFS source code to see how they solved a particular problem and then goes and solves it the same way that residual knowledge might cause legal problems.
That being said IANAL and have no idea whether residual knowledge has been explored w.r.t. the GPL or CDDL.
That being said IANAL and have no idea whether residual knowledge has been explored w.r.t. the GPL or CDDL.