Where GPLed software (no longer protected by copyright) is modified and distributed by a large company for profit, righteous hackers reverse engineer the program and bad mouth the company publicly, so it's generally seen as not worthwhile.

Note that the GPL does allow you to modify and distribute your program for profit, you just have to release the source code and cannot slap additional restrictions on it. So the sin (contra-normative but legal behavior) in this case would be for the company to sell an obfuscated or compiled version without providing source, keeping the source code secret and forcing all programmers who work on it to sign confidentiality agreements as a condition of employment.

It's a good question as to whether this particular practice would remain against social norms for long if it were not illegal. To what degree does the legality of something impact its social permissibility?