A Minimium Wage Employee
I don't know her gross wage but the numbers quoted sounds like minimum wage to me, about $10 an hour. At that wage level her deductions should be minimal. Housing is expensive in the Bay Area, food's about average for California and there is excellent public transport at prices which would make the typical Londoner weep for joy.
Minimum wage is a reality for many people, and not just young people. Its not quite enough to live on which is why there's a push to get it to $15 (which will move it from 'wholly inadequate' to 'not quite inadequate'). Its probably a bit of a shock for someone who was raised in the 'burbs in a nice middle class household.
The reason for the backlash is that most people know the score. They know that many of the people she interacts with daily -- janitors, food service, supermarket workers, transportation -- will be on similar wages, with similar levels of (non-existent) job security. Its not that they don't have sympathy for the low paid -- there but for the Grace of God &tc. -- but rather the way that the open letter screamed 'entitlement', the feeling that as soon as Tilia got what she wanted she'd turn around and look down on their inferiors just like the rest of the entitled.
Incidentally, she might want to look into the pay and conditions that new teachers get in California.