That's because you aren't rolling 0 dice, you're rolling 2 dice. What you're asking for doesn't really make sense. When you are rolling for an action with 0 dice you roll 2 keepnlowest, but if something gives you a bonus die you no longer have 0 dice, so you roll 1 and keep the result.
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Per default that is true, indeed I forgot to mention that I'm talking about a special case.
When you get bonus die through help or abilities you would just roll 1 dice instead of 2 keep the lowest. But when you get bonus die because of additional threats, you would have to roll 2 keep the lowest to generate one outcome and rate the bonus die seperately to allocate to the threats.