Well the relevant difference is whether the shooters had external support to commit their crime.

That it's possible for a determined group to acquire guns and attack people is one thing. It being possible for a single mentally unstable person to do so is quite another.

Feel free to share your sources establishing the shooters of both the Paris Hebdo and Bataclan attacks received external support.

What about Columbine (multiple shooters, conspired in advance to commit the attack), terrorism, mass shootings?

I'm not saying anything about any particular shooting, I'm saying that's the relevant difference.

Actually I think the current practice is to label pretty much everything terrorism. Part of the perpetual crisis strategy used to justify the militarization of police forces.