Given the amount of disinformation put out there by so many sources versus the relatively limited resources of Snopes, wasting time on stuff no one needs or wants is a mistake.
I'm wondering if a better model would be heavily moderated, crowdsourced fact-checking, like wikipedia.
And how would you propose they do that? If there's no article that addresses the specific article, nothing would come up on a Google search for that which means there's no prompt to redirect them. They would still have to create some kind of content that gets indexed which would serve as the redirect.
As a start, need to get more complex from there (probably exclude theonion's About page, for example, but that covers 90% with almost 0 effort -- and standard analogy from there, if Snopes uses 90% of its effort to cover the first 90%, there's no way they can cover the last 10%; if they use 1% effort to cover 90%, there's still a lot of ability to cover the remaining 5% or 9% (because, let's be realistic, 100% is absolutely impossible).
There's a very good reason that people going to Snopes for claims originating in such sites aren't redirected that way, and that's because the claims often get separated from the originating site as soon as someone believes them and decided repeating them as original(or in the form of an unsourced meme) is the best way of sharing them.
Of course, the Snopes article on the claim should mention the satirical original source, but that's no reason not to do everything else Snopes would normally do to address a claim about which they have received questions.
Snopes, and anyone else actively trying to combat disinformation, absolutely have to use their resources wisely, the enemy has a lot more money and time.
Most traffic to Snopes probably comes from people searching for specific articles, quotes, and/or stories and a generic "these are all satire sites" page wouldn't pull up for those. If the goal is to combat specific instances of inaccurate information, even if the intent initially is satire, then what they're doing seems like a better solution that what you've suggested. I can't even understand how you'd jump from that to calling them dishonest because of it.