I guess if you have 100% trust in government to never be corrupt or not make mistakes.

But, if you believe in innocent till proven guilty than this becomes a problem.

Sure, there's probably a lot of very suspicious people on the terrorist watch list. But there's probably people that are on there or maybe on there in the future that are completely innocent and how could you take rights and restrict somebody that hasn't actually committed a crime? So you're saying if the government puts you on a list, that's it.
Basically punishing people that may be innocent.

The criteria for getting on the list seems super vague, so it's easy to see how many innocent people are on the list and of course without even knowing.

Aside from that, if the government actually had evidence that a person truly is a danger to the public, and are in fact a threat, then banning them from buying firearms legally isn't enough to stop any terrorist attack.

So Aslong as the terrorist is free in society, they will still eventually find weapons, guns, bombs, box cutters, whatever and carry out an attack. If they are truly hell bent on it.

It's not like they will show up to the gun store, get denied and then give up for good.