Maybe they got to the technological level for Kardashev Type II, but due to depletion of Earth's (accessible) fossil and fissile resource were unable to make the jump. Thus, they time-travel back to where reserves were adequate, "fix" (not ruin, from their perspective) the environment to their standards, and then…

If using your computer doesn't give you enough redundancy, you clearly haven't enough computers. :p

If using your computer doesn't give you enough redundancy, you clearly haven't enough computers. :p

The lesson is probably to quit using dinky streaming appliances and plug your TV into your computer.

The lesson is probably to quit using dinky streaming appliances and plug your TV into your computer.

Spartacus chops face.

Spartacus chops face.

What, there's others? I'm not alone!

What, there's others? I'm not alone!

Captain Teashades is awesome.

Captain Teashades is awesome.

Because who wants sex in the gutter when you can have sex in a book club?

Because who wants sex in the gutter when you can have sex in a book club?

Because if Kenn Viselmann doesn't teach America's brats to yell at the screen like ninnies at a Tyler Perry film, who will?

Because if Kenn Viselmann doesn't teach America's brats to yell at the screen like ninnies at a Tyler Perry film, who will?

Or "walk around with it, let's fuck my niece"?

Or "walk around with it, let's fuck my niece"?

Her name is Spankopedia.

Her name is Spankopedia.

Yeah, as @drdarke:disqus  says, the industrial base is a problem, so it's perfectly plausible to say virtually all remaining functional semi- and full-automatics, and the remaining stock of pre-apocalypse ammo, are reserved for dashing officers and the occasional military campaign, not for grunts oppressing civilians.