Because a criminal clerk in North Attleboro said one hadn't been signed.
According to the press, right? Can't pick and choose what they are getting right and what they are getting wrong.
I think the truth is most likely somewhere in between. The police probably were at some point considering arresting Hernandez for obstruction, whether by warrant or otherwise. A source or two gave that information before the police got a lid on press leaks, and the reporters went off half-baked, either misunderstanding or making assumptions.
I should probably shut up, because the last thing any of us needs is more idle speculation, but to me, if it's the case that the police were considering an obstruction arrest, but decided to table it while they did more investigation of the murder, including a search of AH's house, it's a bad fact not a good one.
To sum up what we know:
1) A murder victim was found near Hernandez's house. There seems to be good evidence that Hernandez at least knew the victim. There may be a more serious connection in terms of whether they were together close in time to the murder, but this is unclear.
2) Hernandez as of this moment has not been arrested. Good fact. There are lots of reasons why this could be the case and he could still ultimately be charged with a serious crime. But, every day that goes by is good for him.
3) A search warrant was executed at his house, and the police removed lots of evidence, which suggests strongly the warrant was issued to look for evidence of a crime more serious than obstruction. This is a bad fact. No two ways around it. One can make up imaginary ways this is not a bad fact, but it's certainly not a good one.
4) There likely was a destruction of potential evidence. By whom, and why, and what exactly happened is unclear, but the early reporting on this looks solid enough that I believe it more likely than not is true. Without knowing more, there not much to say. It's a probable bad fact, but it requires too much speculation to put it in the same category as 1-3.
5) The police have executed other search warrants related to the murder. Without knowing more, this is a neutral fact.
6) There was something that led the press to go with a story suggesting the police had probable cause to believe Hernandez had engaged in the crime of obstruction. This one is too tenuous to draw any conclusions at this point, although above I gave my entirely uninformed rank speculation that this could be bad.
Other than that, all the rest -- at least in my opinion -- is noise.