The answer - because it is a crucial ideological component of their world view-is worrying enough.
But now we need to start asking other questions
"What would a Lab government led by those who have clearly demonstrated the centrality of this hostility to Israel/Zionism/Jews actually be prepared to do and what would they not do?"
To help inform these discussions we should really begin to seriously look at what we think a Corbyn govt might actually do and what the impacts might be.
The responsible thing now is, i believe, to start on a realistic assessment of what the risks might actually be.