Can you cite examples of them dropping cases against Chinese defendants because of evidence they had to exclude due to it being fruit of the poisonous tree? Inability to secure a conviction can happen for many reasons.

The FBI may very well have been overzealous here (especially considering the history of the organization), but I think we'd need to look at the facts of each case in detail, rather than only the charge and conviction statistics by nationality. The case with the Temple professor appears to have resulted from a misinterpreted email.