By moving to IE9 he's discounting all corporate customers who haven't moved to Windows 7 yet. This leaves them with only one option if they want support on a long-term supported browser, which is the recently shipped Firefox Enterprise Support Release.
Better framed: he's discounting all the corporate customers who drag their feet technology wise, and will give him more headache than money.
Such corporate late adopters are not usually the ones to use something external, much less Basecamp, anyway.