Honestly, it is not all that uncommon for companies to use PTO like that. Vacation and sick days are both considered to be simply 'time off' in the payroll. Though, in most cases, if it is something severe, like a broken bone, they will at least put you on 'light' duty when you return.
As an actual working person, I can confirm it sucks to use vacation time for sick days. That said, it is better than not getting paid at all.
'Sick days' no longer exist after you get out of school. <insert reality disappointing thanos meme>