I thought the boat was on a trailer.
For me I left Ben with the boat so it made sense to me to him getting it taken from him. I don't think it's so much poor writing, it's a poor option to allow more than 1 person to say behind to guard the boat.
You should leave 1 or zero people and have the boat taken. Otherwise it gets odd story-wise, but if you don't let people have the choice it's a limiting factor.
However because no one saw Vernon and his group with guns didn't mean they couldn't acquire them as they were planning to take the boat. Vernon could have been working with Lee for the sole purpose of doing something like that. After Crawford was found to have fallen perhaps that left the a power gap of sorts and let Vernon do what he had always hoped he could. Get armed and get the hell out of dodge.
Crawford was the only thing originally keeping them all pinned down in there safe area after all.