The bass is pristine. The shop doesn't say he used it during a tour or recording. If he had, they would have said so, as that obviously increases the value. There is no question that Gilmour's "Black Strat," which he used extensively in the 70s, is worth more than an essentially identical signature version that he kept as a backup.
It's only a scam if they are lying about its provenance and/or usage.
Dylan sold his catalog last year, possibly for as much as $300 million. He's not scamming anyone with this. I'd guess he gave the bass away years ago, and it's a few steps away from his ownership now.
I see no indication that society's ethics about collectibles have gone downhill in the past decades.
There's an entire market of people who purchase collectibles they don't plan to use for their original purpose. The guy who bought McCartney's Yamaha bass for nearly $500,000 isn't planning to play it in his dad band at the local dive bar. He's going to add it to his collection.
Since there is no evidence of deception or fraud, I really don't see why you object. If someone asks a steep price for a collectible, and you have no interest in collectibles, then what's it to you?