I suppose it is possible that some faction inside Twitter is doing this, but it isn't official policy, and as such they aren't able to delete everything critical of United.

I, however, find it more likely that this is just a technical issue. As when an "eventually consistent" distributed database takes longer than expected to be consistent, if it does at all. Most of the distributed database products out there have lots of edge cases where they are not consistent, reliable or durable. And Twitter is at a scale (size of DB, update volume, number of datacenters) that few others approach.