And specifically what facts do you have to support your assertion?
Clearly, if they had a probable cause to suspect he was involved in the sniper shootings they would have gotten a warrant. Warrants are issued on probable cause. If they thought this individual knew something about the sniper incidents then they would have asked him questions that did not have to do with a rifle purchased yaers ago.
The agents clearly were chasing a wild goose and the fact that they would go after people just because they happened to have purchased a firearm that fires the same round the shooter used in this case. Clearly their time would have been better spent checking out field reports from officers who stopped vehicles after some of these shootings and looking for the people seen in the vicinity of more than one shooting. That might have led them to John Mohammed and it was a valid police procedure. Checking out those kind of details is what led to the arrest of David Berkowitz. It is a tried and true methodology.
Further, unlike the sweep of Maryland gun owners, it does not damage the FBI's reputation for the future. It encourages cooperation in future cases. It does not leave the impression that the FBI is eager to violate the Right to keep and bear arms.