I'm sorry to hear about your experience. Airbnb's policy states that if a guest, their invitee, or pet causes damage during a stay, the guest is responsible for covering the costs. However, damages resulting from normal wear and tear are typically not the guest's responsibility.
In your case, if the bed frame broke under normal use, it could be considered normal wear and tear. If you believe the charge was unjust, you can appeal Airbnb's decision. Additionally, sharing your experience through a review can inform future guests.
I hope this information helps clarify your situation.