It is sometimes the trouble, like with Sonyâ€™s XCP debacle, but also as mentioned because of the price. Once a game has been relegated to the bargain bin, there isnâ€™t much point to copy-protection.
The first time I heard about this sort of thing was when id removed the online authentication for Quake III after it had been out for a while (and yet they still charge for Keen, Wolfenstein, Doomâ€¦. ) Also, Curious Labs removed the Pace anti-piracy protection from Poser 5 when they released SP3. Rockstar went further by not only removing the protection, not only releasing itâ€™s older games for free, but updating them to work on todayâ€™s computers as well.
When a new version comes out and there is not much more money to be had from the old one, some companies have enough sense to remove the protection and reduce customersâ€™ hassle and frustration.