1) It's a new service. It's reasonable to restrict your audience at first. Making it more international requires more work (shipping, billing, etc) which might end up being useless. It makes sense to test with domestic users first.
2) You should take a lot of care before shipping seeds internationally. Some countries (well, all countries I have flown to) have strict regulations on importing seeds and plants.
Bigger companies have pulled this off so many times, restricting their customers to the US for seemingly no reason, that we get irked by it now.
If there was a message saying how it's super complicated to send seeds abroad, then we'd understand and all would be well.
I do care about non-us folk. One of the main reasons why I built the software the way I did (using weather data to calculate planting dates) is that I can theoretically extend the site to anywhere that I can get weather data for.
My other post below explains things a little, but support for other countries is definitely planned.