Yes, you could.  I did, several times, and no one ever arrested me or anything.  However, there came a time when they worried about too many nutcases wanting to take a hammer to it (this actually happened 

 ) so they restricted access to formal occasions.  And then, of course, the National Constitution Center was built at great taxpayer expense to house the Bell behind a bulletproof glass partition.  
 
Classic case of a few bad apples ruining it for everybody.  
