1. No, that's an analog signal. See below for more detail.
2. Yes, that's legal. Worth a try ... But more than likely, you would get the same result.
Without knowing exactly what Tivo is responding to ... and exactly what's getting inserted into your signal ... hard to know for sure.
However, my GUESS is that Tivo is responding to a Macrovision or CGMS-A tag.
My GUESS is that it's not Macrovision ... as that's pretty restrictive, would kill VCRs, and cant legally be applied to alot of content.
CGMS-A is an analog system ... some extra stuff in the VBI of an analog signal. Same place where VChip, Close Captioning, XDS, and other additional info is ...
MCE "respects" CGMS-A. There's restrictive settings like "copy never" and "copy once" ... but there's also a "copy freely" setting. MCE forces CGMS-A protected content (even "copy freely" content) to be be encrypted and not allowing you to burn it to DVD. However, under MCE you can transfer the content around.
My GUESS is Tivo is respecting CGMS-A by not allowing you to transfer to PC. Their protection on the PC is pretty weak ... so preventing transfer may be their only (percieved) option.
CGMS-A could be in the original video ... or it may be applied by a cable STB and inserted in the video. Same with Macrovision ...
Again, the above is all just a GUESS. Without knowing what's in your signal ... no clue if it's accurate. However ... it seems to make sense that they might go the same route/protection as Microsoft/MCE.
I beleive there are several products with legitimate uses that have the "side-effect" of removing CGMS-A flags (and Macrovision too).