There are various explanations for this situation. My preferred explanation is that を can be used with intransitive verbs involving movement, to show that the movement is being done in or through that location.
Similar examples are 家を出る, 公園を歩く, 道を行く. (家を出る and 階段を下りる seem to be movement from a place and I believe that both can be replaced with から.)
Now I'm going to defend this explanation but this part isn't necessary to understand the above. A linguistic category or term has to say something about the structure or pattern for it to be useful. If all your definitions are circular (i.e. を marks a direct object, anything marked by を is a direct object) then you're not saying anything useful about the language. The label "direct object" tells us nothing about how a structure is used. What we want to do is describe a category in such a way that it helps us understand how words in that category can be used (i.e. what other patterns they can occur in).
A transitive verb + object phrase like 弁当を注文する can occur in at least two other important constructions - passive, and -te aru. You can say 弁当が注文された and 弁当が(or を)注文してある.
You cannot say 道が行かれた or 道が(を)行ってある. Therefore it does not seem that we should include these phrases in the same category as ドアを開ける.
Last edited by yudantaiteki (April 19, 11:33 am)