I'd suggest that both your opinion, and the opposite opinion are perhaps further principles that people may have but it wouldn't mean that the person who holds the opposite view isn't being influenced by the practicality of ensuring something they want to avoid. Again I think that looking at this in abstracts is fairly useless. People may say they have principles,, but those principles may be in conflict, or that may think of practicalities and not see things in absolute rules, or a mixture of the two. You still seem to me to be looking on your approach as the correct one, and not accepting that people are messy in their thinking.
Of course I understand that people can be messy in their thinking, but that doesn't really affect the reality that some people will have objections on principle that will not be shifted regardless of the carefulness of the process or the robustness of safeguards in place. I'm not saying that is wrong (and indeed I'm of that persuasion on the death penalty) but it is important to recognise that.
But there is another issue - as with the embryonic stem cell debate, most of this isn't being played out amongst ordinary Joe Public, but largely amongst pro and anti-activists. And those people will use arguments to their advantage even if they might be irrelevant to their actual thinking. So what I am thinking is anti-activist groups that may be driven by people who strongly oppose on principle who realise that shifting ground to arguments (that they aren't really concerned with as their opposition is on fundamental principles) about the practicalities, or the safeguards, or the length of time for debate, or the claimed slippery-slope etc etc, which act to chip away at the other side. Yet no amount of reassurance on these matters will ever be enough for the 'against-on-principle' person.
And there is a further point, specifically the general credibility of the argument - I suspect many who object on principle, particularly if that is based on religious dogma, may feel that were they to be overt about the reason for their opposition that there would be a general dismissal of that argument as the basis for a universal policy which affects those who do no hold to those religious dogmas. As an example AB is against on principle and while we might respect his individual right to hold those principles and to personally act in accordance, I suspect many of us feel that he has no right to try to impose his religious view on others when is curtails their freedoms and beliefs, which may be just as fervently held as his.
And therefore to campaign on 'practicalities' is likely to gain more traction and therefore be adopted by the principle-objecting person but one who is a smart campaigner.