Why does てはいけない use は? I know it's a construction, but where did it come from? て form verbs are not nouns, it doesn't make sense to me that は could be used with them.
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THe は in ～てはいけない. Is actually not the topic marking particle, but a different particle entirely. IT's kind of like how the particle に has a number of different uses. In this case the は is used to emphasize the negative part of the sentence. You'll see in other places が being replaced by は before ない or another negative verb or adjective.
Edit: I found some more information on the subject. This website does a good job discussing this concept as I understand it.