Programmer - [objective-c] why shouldn't I release this string?

Programmer Question

Look at the following method:

-(void)updateProfile:(Profile *)profile WithJSON:(NSString *)JSON;
{
    SBJSON *parser = [[SBJSON alloc] init];
    NSDictionary *object = [parser objectWithString:JSON error:nil];

    NSNumberFormatter *nf = [[NSNumberFormatter alloc] init];
    [nf setPositiveFormat:@"#,##0"];

    profile.displayName = [object valueForKey:@"displayName"];
    profile.profileURL = [object valueForKey:@"profileURL"];

    NSString *rep = [object valueForKey:@"reputation"];
    profile.reputation = [[nf numberFromString:rep] intValue];
    //[rep release];   <-Why not release?

    [nf release];        
    //[object release];  <-Why not release?
    [parser release];
}

I have commented out two lines, which gives me EXC_BAD_ACCESS if not.

Can someone explain to me why it's wrong to release these objects?