[BillyOblivion]

Think about it from the Founder point of view. Very few startups are 8x5. Most expect you to put in a few extra hours, be on call, etc. By letting them know if you've got a side project you plan on dedicating yourself to you are taking those hours out of that pool.

If it's just for an Open Source project, that that's one thing, but if you're also working 10 or 20 hours a week with another company then they (a) need to know that they have to account for that in scheduling and expectations and (b) want to know that you're not working for a competitor and passing inside information.

I had one guy on my team who was working 16 to 24 hours a week at another job, which meant that needed to be careful how I scheduled him (because at the time we didn't have enough work to schedule everyone full time. If we had, well then he'd have to sort that out).