You may want to tak a look at the following document: http://research.microsoft.com/~adya/pubs/icde00.pdf
I think the ANSI SQL-92 standard has taken an approach that will presumably work for a large variety of databases, defining standard isolation levels. The idea being that SQL code could work across all databases. I believe Oracle implements this standard in order to be considered SQL-92 compiant. However, most of us on this list feel Oracle has an inherently better method of handling locking, so ignore this feature.