I can't entirely blame them for this one considering the alternative is either using something standard (localStorage) where they're only going to be able to store a really insignificant amount of information or creating something proprietary for multiple web browsers which is difficult to maintain.

If there was a web standard way of caching 5GB of files locally then I would be annoyed.

IndexedDB is supported on all major browsers by now.

Hmm you have a point though I don't know how well Chrome supported it when they first came out with the offline drive support. May be an issue of legacy needing to be upgraded. I also can't find good performance benchmarks for very large blobs in IndexedDB. But yeah I suppose they could use that now.