It doesn't "trap" Labour if Starmer doesn't set a policy that a lot of his own members and MPs don't agree with.
This slavish devotion and determination to defend Starmer at all costs is odd. The SNP policy on this has been clear for some time, why shouldn't they put their motion before parliament. A lot of people in the country think it's an important issue.
If it causes Labour and the Tories some issues, that's on them not the SNP.
Blaming the SNP for Labour's own failings is obviously partisan, why not just try the shmmeee defence of "it doesn't matter what Labour say anyway". It's not much better, but it avoids having to defend the indefensible.