You are allowed into the US for business on the visa-waiver or the B1 visa (which is easy to get and is the default for non visa-waiver countries I think). But you can't _work_ there. It's a subtle distinction, and I suspect it relies on how the immigration official is feeling that day. Meetings, conferences, meeting investors, networking, etc, are all OK.
Doing the job you normally do isn't allowed, but it seems like it's pretty obvious that people do that.
If you mean do a program like YC, it's fine. The reason it works in this situation is because you aren't making any money. The INS does not care if you are "working" if you aren't actually making any taxable income.
If you mean draw a regular salary, it's harder to do that legally but I have a friend in France who runs a 1-man consulting firm that does short term contracts in the USA and he's figured it out. I don't think it was easy in terms of "no work" but it was just filling out a bunch of paperwork.