The animals one obviously doesn't count [1], since the petition was created September 27th, 2011, and the order halting the use of animals was issued September 20th, 2011.

[1] Barring time travel

I am pretty sure the CISPA veto and the open access rules were only tangentially affected by the petitions. The open access rules were a response to the treatment of Aaron Swartz, which was creating a media storm that was causing more people to notice the general abuses of power by prosecutors in this country. The CISPA veto threat followed pressure from lobbyists and well-connected corporations.

Did the petitions matter at all? Maybe, but they were not even remotely close to being sufficient to get the administration to do anything. The Obama administration already knows how people feel about the Snowden situation; the petition is basically irrelevant.

It could perhaps be worse, either because it makes 100,000 people (or however many) think "well I've done by part now!" so they become complacent, or because the administration thinks "well they've got it off their chest with a petition, that will keep them away from doing anything stronger so let's keep going with what we were doing".

Definitely subjective, though.

The argument is the same, but from the opposite end: Because it costs nothing and takes ten seconds. Why should I care about an opinion if the proponent can't invest more than that into it (say, a phone call to your congressman). What other artefacts of zero-cost, (near-) zero-effort expression should be considered in the legislative process?

It does cost something. It makes you feel like you've done something, so you perhaps have less motivation to do something more substantive such as writing your congressional reps and senators.

Which is a far more meaningful act than signing these useless online petitions.