I'm pretty sure that these guidelines are exactly about protecting against dilution. I think the logic is as follows--the actual Twitter logo (and, consequently, trademark) is not just a bird, it's that specific bird, in that orientation and that color. Allowing anybody to do anything with it would dilute it. And so they must protect against this.
For what it's worth, I've certainly seen documents like this from other companies. Based on these observations (rather than the actual law :/), I think that what they're doing is part of protecting it from dilution.
