Thanks for all replies.
Btw, I found a review of Aldiko
here. When you search the article for "justification", you come across a screenshot just like mine. The author of the review listed "Defaults to Full Justification" as a con of Aldiko.
Then I decided to add a Lonely Planet epub to Aldiko for testing. And there's spacing problems just like mine as well. So, either professional Lonely Planet ebook makers also screwed up, or Aldiko is faulty.
Yes, there are some issues with Kindle as well, but it is not as obvious as Aldiko. I'll try different justification settings at the code and see the results at Kindle and different e-readers.