Or, more exactly, the reasons are purely historical: The method was written as a direct mirror of the one in C++, and in C++ the RowHandle does not have enough information for that. But in the Perl API the row handle object carries extra information for the type safety: the reference to a table that can be handily reused for the other purposes.
This situation has had upset me much, and I've set at once to rectify it. So, now you can call directly:
$rh = $rh->next();
Obviously, it's not in the 0.99 package, but it will be in 1.0.
And while I'm at it, let me also explain why the C++ interface is different. Remember that every row in the table has a row handle. This makes the large tables sensitive to the size of row handles. If a table as a million rows, every extra byte in the row handle means extra megabyte of memory used. Finding the next row for iteration does require a pointer to the table, and that would be extra 8 bytes per each handle. I kind of try to resist the temptations of premature optimization, except where it's absolutely straightforward, and this is one of the straightforward cases.
However in the Perl APIs the objects are not direct references to the C++ objects. They are wrappers that carry the extra information for the safe type-checking (on the other hand, the C++ API is unsafe and assumes that the caller knows what he is doing). Memory-wise this is not much overhead, since normally not many objects are referred from Perl at the same time. And once a row handle is placed into the table, it does not bring its Perl wrapper there with it.