[R] NA, deleting rows

[juli g. pausas]

Dear colleges,
I do not understand the following behaviour:

> aa <- data.frame(a1= 1:10, a2= c(rep(NA, 5), 1:5) )
> aa
   a1 a2
1   1 NA
2   2 NA
3   3 NA
4   4 NA
5   5 NA
6   6  1
7   7  2
8   8  3
9   9  4
10 10  5
> aa[!aa$a2==1, ]  # removing rows with a2==1
     a1 a2
NA   NA NA
NA.1 NA NA
NA.2 NA NA
NA.3 NA NA
NA.4 NA NA
7     7  2
8     8  3
9     9  4
10   10  5

I didn't expect a1 to be affected.
Is  aa[!aa$a2==1, ]  an incorrect way to remove rows? 
Any other way?

(R 1.8.1. for Windows)
Thanks in advance

Juli