I believe there's a few reasons. one is that the google file system blocks that bigtable runs on and "reads" for your query could be on any number of machines. been a while since I read the google architecture papers but I would recommend it to all. it is fascinating how they implemented gfs, bigtable, chubby lock, etc. the work around is essentially to write your own code that does what a database does - something to the effect of getting a list of keys for the records you are interested in and using that to then scan just the area in your data structure you want.
It hasn't got anything to do with that. Creating an index on a join means that possibly hundreds of indexes need to be updated every time a you make a write - but that is where appengine would actually shine since those writes can be parallelized to hundreds of machines. It could easily make postgres/mysql/oracle redundant.
