(Not a teacher)
I believe there isn't a 'rule' as such, no a reason. Perhaps the thing to remember is that, in general, things between 0 and 1 are singular. However, decimals appear to be an exception to this. Perhaps, just an exception through language use, rather than based on rules.
There must be examples that aren't plural in a decimal for between 0 and 1, but most that I can think of are (0.5 degrees, 0.5 feet, 0.5 inches etc).
The point about fractions being singular. You don't say 'a zero point five meter' for the decimal. However, you do say 'a half meter' or 'half a meter'. That is, the singularity is more marked in the fraction form by the word 'a'.
Student or Learner