Asking for all of the IPs that have visited the site is ridiculous. If they want to track down specific users, go for it. Then can go through the site and find those that they think did something wrong.It is a big net, but there are potentially a large number of fish. Recall that over 200 were arrested on inauguration day, so there were many more who rioted and were not arrested.
And, a little more background... soon after the inauguration, the DoJ subpoenaed the same information and was rebuffed. Presumably, some of the 200+ found out they were facing real prison time and squealed that they received their "marching orders" via the website, hence the search warrant this time as the DoJ has developed probable cause satisfying a judge. (if someone has a link to the PC affidavit, please post)
Probably the most relevant information would be emails and lists of people who "signed up" to "man" the various blockades throughout DC on inauguration day. If you've visited the site (I have) you've little to worry about the FBI looking at your IP address... if you "signed up" to "man" one of the blockades where violence and destruction occurred, you might expect an interview.
This is no more OK than any other government program where I shouldn't be worried if I didn't do anything wrong.