You could get a ham sandwich through FISA court, obviously. I don't think anyone is arguing otherwise. It's gross and disgusting and needs to change. But that is not the story.
The story is that the FBI is supposed to be apolitical and supposed to have clean hands when it applies to FISC. In this case it was blatantly neither. While I would hope this would lead to a sweeping reform of the FISA system generally, I'm not going to hold my breath on that one. Even so, Strzok and Ohr both clearly violated the FBI's code of conduct.
Furthermore, regardless of whether there ever was "collusion" between President Trump and Russia, this investigation started based on a calumnious report that was supported by itself, essentially. The entire accusation and subsequent investigation was generated out of nothing, and they knew it was generated out of nothing. Steele's report was supported by the Yahoo news story, which was the result of Steele talking to Yahoo. Some of the FBI team knew that and didn't care.
If Mueller gets someone on "obstruction of justice" on the basis of a bad faith investigation I would certainly argue "fruit of the poisonous tree" on appeal. You, FBI, made up a story. You knew you made it up. You then got Congress to authorize an investigation on the basis of lies and you charged my client with obstructing your spurious and illegally initiated investigation.
ἡ δὲ κἀκ τριῶν τρυπημάτων ἐργαζομένη ἐνεκάλει τῇ φύσει, δυσφορουμένη, ὅτι δὴ μὴ καὶ τοὺς τιτθοὺς αὐτῇ εὐρύτερον ἢ νῦν εἰσι τρυπώη, ὅπως καὶ ἄλλην ἐνταῦθα μίξιν ἐπιτεχνᾶσθαι δυνατὴ εἴη. – Procopius
Ummaka qinnassa nīk!
*MySmiley*