Negan's got that strange sense of sick honor that some TV/movie psychopaths have, and the two people who he specifically ruled out were Glenn and Carl (Carl in that he said to cut out his eye and feed it to Rick if anyone got out of line). When Glenn freaked out about Maggie, he said "I'll give you a free pass on that one." In his twisted logic, it might seem like he was going back on giving that free pass if he subsequently killed Glenn anyway. He thinks by being honest to the group that that makes him at least slightly more reasonable to the group despite the horrors he's going to visit upon them, like "But at least I never lied to you."I think it's very, very, likely Glenn will meet the same fate as his comic book counterpart.
Besides that, everyone was originally expecting Glenn to die here based on his fate in the comics, and they've already done two ridiculous fakeouts on his dying or nearly dying this season, so to do it again would just seem to be stretching audience patience.