Amazon is already a maze of separate entities for tax purposes, for example to tap-dance around 'nexus' standards for assessing sales tax.
Much more simple (and thus more likely) explanations for this 'Seesaw LLC' registration entity are thus:
(1) this allowed them to obscure the filing (especially as of the earlier, foreign, 'priority date') from people searching for Amazon registrations
(2) it allows some tax advantages from cross-entity IP licensing
I'd go with (1); clicking through to the registrations shows the marks first registered in Tonga ('Fire' in May 2011; 'Silk' a few days before the public announcement). I believe I've heard of Apple using a similar tactic to acquire registered trademarks without tipping off outsiders.