Granted, Costco is suing for a declaratory judgment that they're not infringing in response to a complaint.
To say that Costco is stealing all the R&D that went into the balls is rather laughable. Acushnet is known for suing new competitors out of business to protect their own. Beyond that, golf balls are not nearly as high tech as modern electronics so it's not like there's multi-billion dollar fab investment or expensive rare earth metals involved.
If they have patents that are actually being infringed upon (and not nonsense stuff like omg balls with more/less dimples just to renew an expired patent) then costco should lose and pay up. The fact that costco sued for a declatory judgment tells me that's probably NOT the case and they probably ARE in the clear...and they probably don't want to deal with someone suing THEM and seeming like the bad guy so they're doing it first.