Hi guys,
I work for a music publisher, so I have some inside knowledge of what's going on in this field.
Bassically, according to copyright law, it's illegal to give away transcriptions, whether standard notation or tab, of any copyrighted material without the explicit consent of the copyright holder(s). In most cases, the copyrights are held by the labels/publishers, who will distribute royalties onto the artists/writers (after taking their cut, of course). Copyright law states that the copyright holder is entitled to receive a royalty for every single copy of the copyrighted song, whether a recording or a writen transcription (eg standad notation, tabs or even just lyrics) in circulation. This is why it's illegal to photocopy books of sheet music (or any books at all, basically), or make copies of CDs etc. These sites that offered free tabs of copyrighted material have always been doing so illegally - the diference now is that the labels have finally got the powers that be to agree to clamp down on them.
The reason, I suspect, why The Grateful Dead's material is still available is that the band retained the copyrights to their own material and have stated that they are quite happy to let transcriptions be freely distributed.
I realise that this new move will upset a lot of people who are used to finding tabs for free, but as a songwriter myself, I can understand the reasons behind it. Songwriters and composers invariably put a lot of effort and passion into their work, and it seems only right that they should be paid for it eventually if it turns out to be popular.
I can recommend
www.sheetmusicdirect.com as a vey good place to find tabs legally.
Karl