Yep, working in the US for even one day could in theory create a liability to US tax, and the need to file a tax return for a non-US person.
I've seen this in practice a few times - but of course most people just ignore it as they either don't know that this is the position, or they feel that they are paying tax in their home country and therefore won't need to pay again in the US (which if they are resident in a country with which the US has a tax treaty, is probably right). However, the IRS does still expect to have a tax return filed even where no tax is due - to make the treaty claim. Which would mean that the non-US person would need to get an ITIN - which can be very difficult - so again, many people are put off by this and don't bother, rightly or wrongly.
I know of at least one large bank, on advice from one of the Big 4 accountancy firms, who is advising it's European-based staff who visit the US for business, for any amount of time, that they must file a US tax return. Sledgehammer to crack a walnut perhaps, but technically it's almost certainly right.
So to answer the question a little further up the thread - the IRS isn't really treating business trips for US people any differently to how it treats them for US people - as US source income, and potentially taxable without the FEIE or FTC's available.