Interesting that YC is willing to toe the IP theft line on this one. I think plenty of us do in fact realize that homegrown corporate ideas / apps could likely be turned into external new businesses, but then the blurred ethics and legality of doing so occurs a few thoughts later.
An F100 I worked for had an entire corporate group for the purposes of spinning off their IP so that it could be done ethically/legally and give the employees' new startup the boost it needed. Several of these startups have gone on to $MMM/$B valuations. If you're at Boring BigCo and thinking of ripping one of their developed ideas for a small YC check, I'd advise against it.