The weapon in USA (especially long range weapon like rifles) is mostly useful for mass shootings.
Also, terrorism is kind of illegal already.
I just think it is really ironic.
Of course you are correct that any one individual person could not effectively defend their home against a more well armed government dedicated to defeating them, but that in no way refutes my point.
You can remember American Civil War for example. There were people opposing the government, they had weapons but it didn't help them.
There may be cases when weapon can be necessary: for example if you live in a remote uninhabited area. But I don't think people should have a long range weapon when they are in a densely populated city.
Regardless, they don't care: if you take out a few agents they'll just send in more.
To give an extreme but not unprecedented example, if the government wants to wipe out all citizens of $ENEMY_TYPE_X, they might be at least slightly less willing to do so if the agents contracted to enter the homes of the citizens in question know that they are likely to face armed resistance.
EDIT for clarity: I of course do not mean that any one individual would be able to defend their home against a powerful government dedicated to destroying them.
I'm just saying that all else equal, a government considering whether to attempt to wipe out some particular group is more likely to think twice if there are likely to be costs involved in doing so.