Assignment of individual values to data frame columns: intentional or unintentional behavior?

Dear developeRs,

I have just discovered a strange feature when assigning some values to
columns of a data frame: The column is matched by partial matching (as
documented), but when assigning a value, a new column with the partial name
is added to the data frame that is identical to the original column except
for the changed value. Is that intentional ? An example:

On Thu, Aug 5, 2010 at 12:24 PM, Ulrike Gr?mping
<groemping at bht-berlin.de> wrote:
  

Dear developeRs,

I have just discovered a strange feature when assigning some values to
columns of a data frame: The column is matched by partial matching (as
documented), but when assigning a value, a new column with the partial name
is added to the data frame that is identical to the original column except
for the changed value. Is that intentional ? An example:
    

Note that the lack of partial matching when performing assignment is
also documented.

See second last paragraph in Details section of ?Extract
  

However, given the documentation that partial matching is not used on 
the left-hand side, I would have expected even more that the assignment

sw$Fert[1] <- 10

works differently, because I am using it on the left-hand side. 
Probably, extraction ([1]) is done first here, so that the right-hand 
side won. At least, this is very confusing.