If someone takes a photo I own and post on my website and reposts it to pinterest, yes that is violating my copyright.
Pinterest, like most sites hosting UGC is protected under 17 U.S.C. § 512(c) DMCA safe harbor laws.
The majority of pins on Pinterest are from the “pin-it button” a small snippet of JavaScript which content creators embed themselves on websites. Pinterest offers opt-out mechanisms but the majority of content creators see huge value in the additional traffic generated from Pinterest.
You're right it's only "possibly" infringement - there are cases where it wouldn't be (fair use) but the default is that generally speaking, it is, and I would be surprised if more than half the content on pinterest was there legally and with consent of the copyright owner.