I think is by desing, And anyway, I have plants outdoors but under a roof in real life and you have to care for them as they where indoors. They don't get watered by rain and the temperature indoors/outdoors isn't really that different. Indoors plant can also get bugs if you are not careful.
I mean, If the game alows you to keep summer plants indoors without a true heating/insulating system, or without proper sunlight. Then you should't be able to cultivate summer plants all year round regardless of walls. It's equally fantastic.
