These games are still copyrighted and LucasArt is still hunting pirates over the net.
You must buy it in a 2nd hand shop or on ebay...
I doubt they are 2 reasons:
1: They don't sell it any more so they would not be loosing out if everyone on the planet now copied it.
2: You would only spend time and resources on anti-copyright infringements if you have a commercial interest to do so. As they do not make any money from the Amiga market I see no reason why they would spend money looking for copies of their software.
If you buy a real copy of classic Amiga software on eBay you are not supporting the industry any more than downloading it from the Internet. There is no market so I recon you could write a letter to Lucas Arts, informing them you have an illegal copy of Monkey Island and they wouldn't give 2 hoots.