Because its not the apps thats at fault, it's the user who must make a conscious decision NOT to download illegal programs,
don't blame the software, blame the users for making these program notorious.
There are many great uses for P2p applications, even alot of GNU/LINUX distros which are 100% legal to download, are only being made available via bit torrent. as it saves on bandwidth costs.
Cassette tapes were often used to record illegal copies of music, but many people did use it for legal purposes, such as medical transcriptions, old data drives used cassette tapes. Sell tips were placed on them, as well as unsigned acts would record their demos on cassette tapes.
The technology is a great thing, dont blame it, blame those who abuse it.
You only will be sued if your downloading illegal content. If you download legal music or freeware or shareware on these apps, then you can not be sued.