It very much is, and it's legally called "intimidation". On top of that, one of the charges against Stone was "witness tampering", which in this case also involved alleged "intimidation" of a witness.
Even some of Trump's antics could have legally been prosecuted if he wasn't in office as he also used such tactics-- and he still is.
Yeah, drawing a bullseye on a picture and posting it online isn't getting on the phone and killing someone, or is it even implying that someone should.
To tamper with a witness, you have to have a case... There is no Russian Collusion, so there is no case. Does anyone find the irony in prosecuting a guy for interfering in a case that already folded up and failed? The "witness" also said they didn't know a damn thing about it, and also didn't see Stone as a threat. So, what actually happened here?
1) Stone is guilt of tampering with a witness that didn't know anything about a case that no longer exists. To be a witness, you have to provide testimony/evidence. Since no evidence was provided there is no witness to tamper with.
2) A distasteful meme.
3) Stone threatened some people through dm's or something, but those people just thought he was talking out of his *** and blowing off steam. They told the prosecutors / judges the same, and they have no grounds to prosecute that. No evidence has been provided to the contrary.
4) Stone was in the media and reaching to Wikileaks to get scoops (like many other journalists do). He got some tips that some releases would be made as did many other notable people and shared that info with his personal friend Trump. Nothing about this is wrong. It's likely that other media were doing the same for Hillary.