"Is writing a piece of software that scrapes the yahoo web page and provides you as the user with a different interface in violation of this?"

Yes, that is the right line and the talk about reverse engineering applies as well. They clearly want their customers using only that interface which allows them to finance their service through advertising.

"In a sense, you are still using their interface, you simply added a proxy. It isn't technically much different from a blind person using sofware that reads the contents of the web site to her."

Except that the person involved is not blind and is trying to bypass Yahoo's interface :) I understand the point you are trying to make, but the TOS seem pretty clear to me. I'm sure they put that last sentence in to specifically address the issue of web scraping.

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Allolex