In the former case, the miRNA is being targeted toward the lncRNA to induce RNAi of it. In the latter case, the lncRNA is acting more like a sponge, but complementing the miRNA and thereby preventing it from binding to its normal target. In reality, both could and likely do occur at the same time. It would be unsurprising to find that a lncRNA has some notable function in and of itself (thus making it an interesting miRNA target), but that the same transcript could also act as a sponge for miRNAs, preventing them from interacting with other targets. Biology likes to be messy like that.