My reading of this poem makes me think these stars are witnessing something tragic, that makes them throw down their spears, like soldiers disgusted by their cause. Of course they were crying, that is just typical ;)
The poem asks what "frame"s the Tyger, I read that as create or capture the soul of this fearful tyger. I think: wild being tamed, nature being constrained, and I think the poem is lamenting a loss of wildness. As if defining something as wild actually limits it. The lines following your interest were what caught my eye: "Did he smile his work to see? / Did he who made the Lamb make thee?" (19-20). I won't go biblical on you, but its there.