While it is usually just a guideline for other sorcerers, it seems Philomela has literally been conditioned to be unable to accept anything to eat or drink from others. That is why she threw away the tea Chise gave her.
That is also probably why the potpourri Chise gave her seems to be so special to her, since she was actually able to receive something from someone that she didn’t feel compelled to throw away, since it wasn’t something to drink or eat.