They are likely banking on IE6 users also often have FireFox installed too. Chances are they won't loose any usage over this and I believe that 37Signals has an amazing ability to generate hype they will see more customers not less.

Myself, I have a lot of nearly computer illiterate people using my system with over 30% IE 6. If I can't get something to work in every browser that people use I just don't use it.