First of all, Sensi seeds does NOT list Jack Herer as "indoor only". They say:
http://sensiseeds.com/cannabis-seeds/sensi-seeds/jack-herer
Then they explain:
Sunny / Mediterranean
Strains marked with this icon can be grown outside in a warm, sunny climate with a long summer. In the northern hemisphere, many locations below 45N and all locations below 40N will provide an excellent climate for any cannabis strain to grow outside. In the southern hemisphere, all locations can be considered (at a minimum) warm and sunny.
So you CAN grow it outside, so long as it doesn't get cold too quickly in your location. And people definitely DO grow this strain outside.
In general, any plant that can be grown indoors can be grown outdoors, they just may not be quite as suitable due to reasons of poor humidity and/or pest tolerance, etc.
One BIG problem is that longer flowering strains (potentially including Jack Herer) need long flowering times, and won't mature outdoors until mid-October or later. At that point the combination of cold and rain will make many outdoor plants rot-susceptible, and on top of that, its a race to finish the plants before winter frosts come and kill them.
So I think these last two things are reasons why Jack Herer *may not* be suitable for various locations.
If you're in a warm place like the Southern USA or CA, or such, then you should have no problem growing JH outside. If you are in a more cold Northern climate, there are still things you can do, like grow the plant in a greenhouse, or put it in a pot which you take inside if the weather dips below freezing.