Why doesn't the "Fruit Of The Poisonous Tree" doctrine apply to the Steele Dossier/Russia Probe?

via Daily Caller.

Thanks to the Department of Justice Inspector General’s report, we now know for certain what has been, for those paying attention, fairly obvious. The Steele dossier played a central role in the genesis of the Russia hoax and was used to justify extensive spying on former naval officer and Annapolis graduate Carter Page.

The top two leaders of the FBI were closely involved in this fiasco. Other powerful people knew what was happening and lied to cover it up. That all was confirmed by the IG report. The report was a disaster for the credibility of top leaders in Barack Obama’s FBI, and it’s also a big problem for the American news media. 

Here. 

This is just a little late Friday night musing.  Why doesn't the fruit of the poisonous tree apply to the Steele Dossier/Russia Probe?

If the initial reason for the investigation proved criminally obtained/false then how can they have moved forward with this probe?

I understand how an intel probe can continue.  They operate outside of regular rules of the law...and for good reason.  I understand that Congress has a role to play in intel (they're the money for the agencies afterall).

But they're moving this over to the quasi-criminal.

How is that allowed?

Yeah its a basic question but dipping my feet into some new stuff and a little knowledge is dangerous...so hopefully the good folks here will expand on my little and make it, if not a lot, then a bit more than I have right now.