Laws are also designed around intent rather than outcome. So there are harsh punishments for intending to start/spread any kind of fire, even if it looks unlikely to go out of control in this particular case.
Also, it's unlikely that she'll really get 10 years without someone being hurt in the process.
If I were to highlight excessive penalties, I'd probably point to drug crimes rather than arson. I don't think lighting fires should be a part of the protest landscape.