Well, if the petition went on to say “…until such time as fail-safe judicial measures can be implemented” then I’d have a hard time signing that petition too. But citing reasons for one’s opposition to the death penalty doesn’t impose a condition on that opposition. IMO, the reason people cite the racial and social bias apparent in the death penalty is because the argument on a criminal’s human rights is often answered by supporters with “vicious criminals deserve to die; it’s justice.” It’s now up to opponents of the death penalty to demonstrate how the DP machine isn’t functioning in a just manner, and instead of the crime being the determinant, it’s the accused’s race and social class. And not only that it’s functioning in such a manner now, but that this bias is an inherent feature in the death penalty machine overall.
Additionally, I don’t think the positions are mutually exclusive. I can be opposed to the death penalty both from a political and a moral standpoint, and therefore call for a moratorium and from there abolition. (As a matter of fact, one of the main slogans in the Campaign to End the Death Penalty is “Moratorium Now - Abolition Next”.) Abolition becomes that much easier to achieve when moratoria start falling into place. Refusing to support a moratorium only makes achieving abolition harder, and thus reluctance to call for a moratorium simply because of a perceived sentiment in the language of a petition makes no sense.
Of course, you could always ask the petitioner if they would support the death penalty given a fail-safe judicial framework, but dollars to doughnuts you’re not going to find very many of them.