But that's not simple, it fact it reeks of disingenuousness.
- it's a terrible deal for the country but even if we could block it, we won't, that's how principled we are.
- it's a terrible deal for the country, but although we can't block it, we'll pretend we can but won't, that's how principled we are
- it's a terrible deal for the country but even though there's no other deal, and no deal is far worse, we'll sit this one out, that's how principled we are
The obvious accusation is that if you don't support this deal, then you support no deal because there is no other deal - you don't get to credibly support 'nothing'. And the Tories get to beat up Labour for being no dealers, anti Brexit, not credible, disingenuous etc. The hair splitting in your position wouldn't hold.
so labour has concluded: 'it's a terrible deal but even this terrible deal is better for the country than the alternative. Therefore we have to support it on that principle'.