Edit2: Thanks for poking holes in this everyone. There are a couple leaps in logic in here that are concerning that I didn't see at first. And the poster definitely leaves out anything about what LE actually does as opposed to what they're supposed to do if they follow the rules. Of course, this was over in the law sub, so the poster wasn't speaking in the context of the DNMs. Even so, thanks for the good discussion.
Considering the Rule 41 insanity, I thought this would be useful information for people to actually understand more about search warrants and probable cause.
I can't take credit for this. It's by a smart person talking sense on another sub. I found it here.
using Tor in and of itself would not be probable cause for a warrant. I think this in part stems from a common misunderstanding about probable cause and how search warrants work. When it comes to search warrants, "probable cause" does not mean "probable cause to believe some person committed a crime." Arrest warrants care about that, but not search warrants (with the odd exception of search and seizure warrants in aid of arrest). Instead, when a court is evaluating the evidence supporting a search warrant, they are looking to see that it provides probable cause to believe that the places to be searched in the warrant are likely to contain the things that the warrant application seeks to seize. What are the permissible things that such warrants may seek to seize? Things that there is probable cause to believe constitute evidence of a crime, contraband, fruits of crime, or property designed for use, intended for use, or used in committing a crime. "Using Tor" is neither a place to be searched, nor a thing to be seized.
Now, that does not mean a person's use of Tor can't be a part of the probable cause determination in a search warrant application, it can be and often is. Driving a car also is, as is using the phone and using the internet. But none of these things, by themselves, would give rise to probable cause to believe a search of any particular place will yield evidence of a crime, contraband, fruits of crime, or property designed for use, intended for use, or used in committing a crime.
Edit: It's one of the responses in this post with a fearmongering bullshit title here. A lot of the responses are very illuminating, actually, particularly from that poster.
The last paragraphs' first sentence pretty much invalidates his prior assumptions.