Where I live, by law employment is "at will", meaning the employer or employee may legally terminate the relationship at any time without cause. That said, I think that being employed, particularly full-time, implies that you will be employed continuously until that employment relationship is terminated. One of the protections that a wise employer offers is that nobody is summarily terminated without some continuance of their salary - two weeks, for example. One of the implied benefits to the employer is that you'll give notice before walking out. In my mind, an employer that furloughs its employees, breaking the string of continuous employment, breaks those implied agreements. An employer that simply dumps employees on the street without continuance makes it so that it is in the employee's short-term financial interest not to give any notice unless they have the ability to immediately start at their new position.
However, if you walk out on your current employer without notice, that is definitely something that the new employer would take note of. The bridge you're burning may not be just with your previous employer.
