Yeah this one really bugs me too. I listen to the EVCast, which is a podcast on electric vehicles. Every so often they bring up global warming, because that's obviously one motivation for people to switch to EVs. But the hosts of the show don't know bubkus about the science behind the issue, and they're in the camp of people who believe that the "doomsdayers" can't be right and the deniers can't be right, and the truth has to be somewhere in the middle. They don't really have any reason for believing this, except that some people just seem to think that on a contentious subject, the truth always lies somewhere in the middle.
Their viewers always take them to task when this comes up, so they've simply stopped discussing the issue. They had a global warming show once, but it didn't really involve any science, basically just brinigng up the various points made by both sides and making jokes and stuff.
That's where this kind of argument comes from, that there's a nugget of truth to both sides and it's probably both a natural cycle but also accelerated by humans. But of course this is a very unscientific position. As those who have studied the issue know, we should be in the midst of a very gradual cooling period according to the Earth's natural cycles, as discussed in the Global Warming Causes Wiki, not a warming period, and certainly not a rapid warming period.
So it does bug me when people make this 'it's natural and accelerated by humans' argument, because they're just trying to find a middle ground, but it's scientifically wrong. And the 'it's all just natural' is even worse, because there's no truth to that whatsoever.
As for science teachers, I suspect they're just trying to avoid having to hear from angry parents. They probably haven't studied the issue in any depth and are taking the least controversial position possible.