I know that some clipart licenses prohibit using their artwork in logos, but not a lot of people/designers are aware of this.
This tree example (if it really was clipart to start with) is a great example of why terms of use forbid use in a logo.
Maybe the hapless designer of the Six Flags logo really did use clipart. The fact that the same trees show up in another logo (that OP posted) might indicate a clipart origin. If so, shame on him/her...
This, most designers don't know that clip art in logos is a no-no unless you purchase exclusivity to the art. Two reasons 1) clipart terms forbid logo use 99% of the time 2) won't ever be able to trademark the logo as it's derivative.
Always a chance Six Flags used one of those designers, happens all the time if no one is paying attention, even in big agencies. In either case, the OP logo needs a redo (which he's said he's doing).