Due to the light fitting, the designer may have bonded the metal door frame because it could be classed as an Exposed Conductive Part as opposed to an Extraneous Conductive Part.I would leave un-bonded provided there was no possibility of any electrical conductor coming into contact with the frame work. Regulations may provide a different answer.
My reasoning comes from some years when a set of historic metal doors to the outside area were bonded to the main electrical earth because there was a light fitting on the door frame.
The definition of an extraneous conductive part is:
"A conductive part liable to introduce a potential, generally earth potential. and not forming part of the electrical installation."
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