While, yes, it probably is more convenient for 37signals than it is for the customers, it would be beneficial for the customer going forward.

There are a lot of CSS hacks that need to be done for IE6, which makes developing annoying and tedious, and takes away time from developing potentially better features.

Also, they can start writing code that capitalizes on better standards support and newer technology that IE6 does not have, giving the users a better app to use.