Strange behavior dropping column from data.table in R

I have been playing with data.tables and noticed some strange behavior and not sure if I am doing something wrong or not.

If I reassign data.table to a different name and then delete the column from the new table, it will also delete it from the first table. For example:

a <- data.table(x=rnorm(10,3),y=rnorm(10,3),z=rnorm(10,3))
> dim(a)
[1] 10  3

b <- a
b[,z:=NULL]
> dim(a)
[1] 10  2
> dim(b)
[1] 10  2

      

        
        
        
      

    

However, if I use the data.frame approach it has no effect. For example:

> b$z <- NULL
> dim(a)
[1] 10  3
> dim(b)
[1] 10  2

      

        
        
        
      

    

Am I doing something wrong with data.tables or is this just a quirk?