Many states -- and I believe Indiana is one of them -- have recreational use statutes to encourage landowners to allow people to use their land for recreational purposes like hiking, riding, hunting etc. The statute limits the liability of the private landowner as long as the landowner is not charging for or otherwise authorizing use of the land (e.g. does not apply to commercial/business establishments). The landowner is usually not protected against injuries caused by the landowner's illegal or malicious acts (so no booby traps or tiger pits), or attractive nuisances, but he is relieved of liability from the (legal) trespasser's own acts. Thus the trespassing laws are written with this in mind, i.e. if you don't want people to use your land for recreational purposes, you have to post it. Purple paint is used in several states as an easy, inexpensive, relatively maintenance way to post the property.
Paint on a tree is a lot more difficult to steal as well. Our No Hunting/No Trespassing signs are always missing and need replaced frequently. I got so tired of buying them, that I started printing and laminating my own. Much harder to remove the paint mark than a sign. We've had signs removed from trees and pulled out of the ground and thrown into the ditch along with the posts they were posted on.