I think acceptable BOSS usage can best be described as "anything that doesn't piss Y! off". Now, this may be a compromise between their lawyers and the host of innovative search offerings they can't put in a legal box at the moment, but I'm still hesitant to think this is all great.
Looking through the TOS I also found this beauty (good luck writing a blog post announcing that you are now integrating BOSS search results):
(a) You shall not refer to the Services, or use or display any Yahoo! trademarks, service marks, logos or other Yahoo!-related branding in connection with any Links or Web Search Results or in any other manner, except where expressly and specifically authorized by Yahoo! in writing.
Yeah, but that's the kind of "just in case" language that's rarely actually used if you're not being a jerk. There's a limit to what's actually enforceable.
If you wrote a blog post stating that you're using Yahoo's new BOSS service in your app, and showed the Yahoo logo there, and talked about how great it is, I sincerely doubt anyone at Yahoo would have a problem with that.
However, if you had an app that was specifically designed to phish passwords, and you put the Yahoo logo on it, and Yahoo sued you for inappropriate use of their IP, you couldn't even begin to claim that they gave you permission. This kind of legal language precludes them from appearing to even accidentally permit that kind of thing. It's just erring on the side of being cautious, which is expected, common, and responsible.
I agree with your interpretation (if you write positive stuff I don't think they'll complain), and I think they do have the right to just arbitrarily decide that someone is doing something bad and cut them off.
I don't like that they are saying that this is an open system, then laying down a legal trap where "everyone" who writes anything about the service would be in violation of their TOS. You can't have it both ways.
They're saying, "Unless we have given you permission to use our IP, you don't have it."
It's not like Yahoo is claiming that BOSS is GPL'ed or anything. And even if it was FOSS, PHP has similar language in its license--you can't use the phrase "PHP" or the logo to endorse or promote any product without written permission.
Language like this in a license never trumps fair use anyhow.
That is a beauty indeed. Instead of trying to describe the appropriate use of affiliation and risk being associated with ne'er-do-wells, they are just avoiding the problem entirely.
"Neither the name of the <organization> nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission." ~ bsd/mit licenses
Looking through the TOS I also found this beauty (good luck writing a blog post announcing that you are now integrating BOSS search results):
(a) You shall not refer to the Services, or use or display any Yahoo! trademarks, service marks, logos or other Yahoo!-related branding in connection with any Links or Web Search Results or in any other manner, except where expressly and specifically authorized by Yahoo! in writing.