subset of matrix vs data frame

In Splus this works,

a <- data.frame(matrix(round(runif(10),0),nrow=2,ncol=5))
a[a == 1] <- 2

but in R only if a is matrix,

a[a == 1] <- 2

Error in [<-.data.frame(*tmp*, a == 1, value = 2) : 
        matrix subscripts not allowed in replacement

Was this a design decision?  Sorry if I missed it in
An Introduction to R.

Denis
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