In this case, where they know the previous block is valid b/c they mined it, why wouldn't they simply mine the next with tx's in it? the reason these spv miners do what they do is b/c they don't want to waste time validating another pools just received block, which is not the case in this situation since it's their own.
It is both as can be seen when 2 blocks back to back come from an SPV pool and the 2nd block has zero txns. The reason is they've not gone to the effort to make the bitcoind validation process fast in their own nodes and they don't want to wait for bitcoind to validate the block (it takes time) before starting on their next block. For solo.ckpool.org and kano.is we run a customised bitcoind which speeds up the validation process dramatically, making this delay negligible.