R: Shouldn't generic methods work internally within a package without it being attached?

I am writing a package that uses classes and functions from the spatial sp library. sp exports methods for rbind (am I correct in calling rbind a generic?).

For instance, the following code creates two SpatialPoints objects and then uses rbind.SpatialPoints to join them together:

> crdsA <- matrix(c(1,2,3,4), ncol = 2)
> crdsB <- matrix(c(7,8), ncol = 2)
> 
> sptsA <- sp::SpatialPoints(crdsA)
> sptsB <- sp::SpatialPoints(crdsB) 
> 
> sp::rbind.SpatialPoints(sptsA, sptsB)
SpatialPoints:
     coords.x1 coords.x2
[1,]         1         3
[2,]         2         4
[3,]         7         8
Coordinate Reference System (CRS) arguments: NA

However, if I then convert the SpatialPoints to SpatialPointsDataFrame (a higher level object class within the sp library), and then use rbind.SpatialPointsDataFrame, I get an error:

> sptsdfA <- sp::SpatialPointsDataFrame(sptsA, data.frame(IDs = c(1,2)))
> sptsdfB <- sp::SpatialPointsDataFrame(sptsB, data.frame(IDs = 3))
> 
> sp::rbind.SpatialPointsDataFrame(sptsdfA, sptsdfB)
Error in rbind2(..1, r) : 
  no method for coercing this S4 class to a vector

A look at the rbind.SpatialPointsDataFrame source code reveals that it calls rbind for SpatialPoints:

rbind.SpatialPointsDataFrame <- function(...) {
    dots = list(...)
    names(dots) <- NULL # bugfix Clement Calenge 100417
    sp = do.call(rbind, lapply(dots, function(x) as(x, "SpatialPoints")))
    df = do.call(rbind, lapply(dots, function(x) x@data))
    SpatialPointsDataFrame(sp, df, coords.nrs = dots[[1]]@coords.nrs)
}

So this seems to be the problem, but I do not understand why. If I attach the sp library, then none of these problems occur, but I thought that since rbind was being called internally within rbind.SpatialPointsDataFrame, then the rest of the library did not to be attached.

Within the context of the package I am creating, even if I include import(sp) and importFrom(sp,rbind.SpatialPoints) in the NAMESPACE, the code above does not work.

I guess there is clearly something I am not understanding with regards to loading, attaching and importing packages. Could anyone explain why sp::rbind.SpatialPointsDataFrame does not work without the rest of the library being attached, and how I can get it to work within my package?

Thanks a lot!

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