Quoting Joseph Hick (leet16y@yahoo.com):

> i don't understand how can the law allow my employer encroach on my
> personal life?

USA law in general permits you to stupidly sign away (most) rights you
would otherwise enjoy via one-sided contracts, and the burden is on you
to realise that they're not in your interest, and that you shouldn't
sign them.  However, there are exceptions (abusive contract provisions 
that are automatically void by statute) in some areas including
employment law; as John Cowan says, the amount of statutory protection
against predatory employment contracts depends on on state jurisdiction.


For California, here's a reference on proprietary inventions agreements, 
covered in Labor Code section 2870-2872 (i.e., by statute law):
http://caselaw.lp.findlaw.com/cacodes/lab/2870-2872.html                        

In brief:  Regardless of what anything you sign purports to establish to
the contrary, whatever you create entirely on your own time with your
own resources is YOURS, not your employer's.  Caveat:  The burden of
proof that you meet those conditions is on you.  (Keep diaries and
changelogs, for example.)


Here are references on non-compete clauses, covered in California by
Business and Professions Code section 16600 (another body of statute law): 
http://caselaw.lp.findlaw.com/cacodes/bpc/16600-16607.html
http://www.fed.org/onlinemag/Feb00/tips.htm

In brief:  No employment-related covenant not to compete can shut you
out of your trade or profession, regardless of what you signed (except
where you owned the business that "employed" you and are now selling it,
or are dissolving a partnership and making such an agreement with your
fellow partners).  Those covenants are legally void.


Often quite different regimes apply in other states.


> what i do in my spare time is my personal life. 
[...]
> please help me understand this.

As one of my personal heroes, the late Rep. Barbara Jordan (D-TX) used
to say, "If you want to play the game properly, you'd better know every
rule."

(By the way, the shift key's on your left.)

-- 
"Zees American words are too much.      Zen our culture you'll wrench; 
With 'le parking' 'le weekend' & such.  Wiz our children we'll be out of touch."
Eef you anglicize French,                -- L'Academie Francaise in a nutshell