It's all the r&d efforts, their Intellectual Property (IP), then the patents granted to protect them, then protecting them.
If apple's patents are being violated then they have to sue, simple.
There is nothing wrong with protecting your IP.
To be honest, Apple deserved to lose the multi-touch patents; and I hope they lose every time they sue a company for that.
But samsung is absolutely shameless. They have copied all iPhone designs from iPhone 3G to iPhone 4; their keyboards; their UI. Samsung is disgusting. If Apple made a friggin refrigerator, they would have copied that as well.