It makes no difference how long you've been with a company so yes, it's legal to lay off any and everyone without severance. The exceptions: a union agreement requiring severance, a personal contract calling for a severance. This is usually only for executives and ``key'' people.
Severance is never a requirement of any employer unless you have a signed employment agreement stating otherwise, or, it is a written policy of the company.
Lump sum severance package cons Lump sum payments may push you into a higher tax bracket for that year. You need to manage your finances more carefully to ensure the lump sum lasts until you secure another source of income. Finally, you forfeit any negotiation power for future benefits or assistance.
The Employee Benefits Security Administration (EBSA) may be able to assist an employee who did not receive severance benefits under their employer-sponsored plan. Please contact EBSA if you have any questions.