"I don't know why people say a double-edged sword is bad. It's a sword. With two edges." -Kamahl, pit fighter
A piece of fine art is more than just a pretty thing to look at. Some artists' works become pieces of history. When people start forging a piece of history it is pretty bad. At it's most innocent a forger creates and sells a copy of an already existing piece of work. At it's worst, the forger creates a new work by forging the style of a respected dead artist and sells it as one of that artists newly discovered pieces. In the latter case you are altering historical records. If you don't find fault in that then there is nothing more to talk about, but I and many other people view such actions as particularly heinous. It should also be noted that there is a difference between a forgery and a reproduction of a piece of art. A good and ethical artist can make good money producing reproductions.
"Conversion, fastidious Goddess, loves blood better than brick, and feasts most subtly on the human will." -- Virginia Woolf, "Mrs. Dalloway"