In the US, in some places, yes.

The post's author is Australian, and is writing from that perspective. We have minimum legal notice periods from both sides (a week for the first year, evenutally going up to four weeks): http://www.fairwork.gov.au/TERMINATION/NOTICE-PERIODS/Pages/...

In markets where employers have a lot of leverage, some employers do more or less do that. They hire more employees than they really need, then put in stringent screening practices to cut the ones they don't want to keep. There are a lot of crappy employers out there, who can get away with it because many people are not in an economic position to object.

In markets where employees have more leverage and can choose which employers to work for, using the legal right to fire people whenever you want to produce a trial period as a matter of routine is a good way to get a bad reputation. If a company is known for firing 30% of its hires within the first 6 months, many people with options will choose another option. Or at least, they will have to be much more attractive in other ways to make up for it (higher pay, more interesting work, etc.). If that's really the intent, making it officially a trial week up front is a lot more transparent.

Yes, but good employers have enough respect for their employees as humans that they want to avoid hiring and then firing someone at all costs.

Not to mention, a company that routinely fires new employees after just a week or two would probably get a bad reputation pretty quickly. Unless they're Facebook (which supposedly does/did this in six week intervals), they probably would take a severe hit on the quality of applicants in the end.

Yup, and you will discover the at-willness of your employment when you ask for a week of personal time. They will panic that you are running off to play some other founders little game.

Only in some countries. In many countries, workers do have rights and can't be fired trivially.