Hey tommo, you are correct- and I understand your concern.
For rank tracking....
Yes, it is 'technically' against Google's guidelines to scrape for rankings, however- Google is very aware of SEOmoz, and has never publicly stated anything against them. (They did ask SEOmoz to stop reporting pagerank a few years back though.)
Although this is anecdotal and doesn't prove an endorsement.... at a lot of conferences, and on some whiteboard Fridays, Matt Cutts has been in touch with the Moz team. This puts me at some ease about Google penalizing all Moz users. After-all, this is an entirely white hat community
As for everything else besides 'rank tracking'...
seomoz uses linkscape, their proprietary crawler and index of the web. This means, whether you are signed up or not, they may still crawl your site (and probably will), because that's how OSE gets data to return in open site explorer.
One final point to keep in mind...
SEOmoz uses several data centers around the US, so using their rank tracking would lead back to them, not you. Anyone could just as easily track your rankings as you could, so how would Google know to penalize you? They really couldn't because otherwise, all you'd have to do is setup a campaign for your competitors and watch their rankings fall.
If you're super concerned, you could use everything in the web app besides rank tracking, and you'd have absolutely nothing to worry about
(and the price would still be worth it- in my opinion).
Does this help?
[edit]
Please Note, I am completely white hat, and if I had once ounce of suspicion that search engines frowned on the rank tracker in the web app, I'd stop using the rank tracker. And that's a decision everyone needs to make for themselves. Above all else, I study the hell out of webmaster guidelines and try to follow every rule and piece of advice that I can.