Not a design problem at all. You just have to learn to push with one leg and lift with the other. If you have standard pedals, you have to stand, but not pull on the bars. You get used to it, and at the end of the day are a better rider because of it.
After a couple of months of riding one, I learned to climb hills on a folder better than I would on a regular bike. And the small wheels help with the intertia problem (I think, though I might be talking out of my bung hole), so it's easier to climb with them apart from gearing.