“Don’t touch it. Rocket fuel is nasty stuff.”
Would not re-entry heat sanitize the surfaces?
I dunno. Maybe these things don’t get as hot because they are kind of buoyant and float down. Or something.
These tanks survive because they are internal to the satellite thus do not take the brunt of the reentry heat.
Even though they do burn through, as evidenced by the photo. there is still some residue remaining on the inner wall. Whether it is Hydrazine or Nitrogen Tetroxide it is very nasty stuff.