Look, the guy saw 15-1 odds and intelligently jumped on them. No reason to hammer on him for it. The Bird, you said you thought going into the season the odds to win should have been about 4-1 for the Celtics (25% chance), 5-1 for Cleveland (20%), et al. That's your rationale for why you don't believe the 15-1 odds ever existed. Well, Simmons is right. Check out this article from June 20, 2008:
http://www.hoopsavenue.com/2008/06/2009-nba-finals-odds/Given those odds (Boston at 7-2, Cleveland at 16-1), if you thought Boston had a 25% chance to win the title and Cleveland had a 20% chance, your expected values for $50 bets on each team would have been $43.75 for the Celtics (25% chance to win $175) or $160 for the Cavs (20% chance to win $800). It's not even close which one a rational person would pick. The expected value is just far higher. It's a no brainer, even for a Celtics diehard. In fact, even if you thought the Celtics had a 50% chance to win it all and Cleveland the same 20%, the expected value would still point toward Cleveland ($160 to $87.50).
Remember, Vegas doesn't base odds on who it actually thinks will win (that will influence the lines, but not control them), it bases odds on who the public thinks will win and who they will bet on. Vegas plays for the long run, they want roughly equal action on all teams so they guarantee a profit (both by selling more than they pay out AND by picking up a lot of commissions).
At the end of last season and going into this one, most people thought Boston, LA, Detroit, San Antonio, probably New Orleans and maybe a few others would be better than Cleveland. Remember, Cleveland had fewer than 50 wins last year and a negative point differential. There wasn't much reason to believe they'd get significantly better (Mo Williams didn't seem like that big an upgrade), and most of their surrounding players were just getting older (Ilgauskas, Wallace, Szczerbiak) not better. They were not an appealing pick from an average person's perspective, at least not as appealing as New Orleans, Orlando and Utah - three young up and coming teams who had better records last year than Cleveland - San Antonio (healthier, bounce back year) and Houston (Artest addition and healthier) or Boston and LA. More money was going on those teams - odds are just shifted to get more money on other teams, like Cleveland.
Factor in that much Vegas action involves teams from California (particularly LA, since that's the bulk of who goes to Vegas), New York and Boston (East coast media hype), so those teams will have much lower odds than one might expect. But they still get more action because so many people from those towns go to Vegas and drop bets. That's a big reason teams from secondary markets, like Cleveland, get higher odds. It's not because they're worse or because Vegas thinks they're worse; it's just because fewer people bet on them so Vegas needs to bring more action that way.
Simmons jumped on what many of us noticed from the Celtics' playoff series against the Cavs last year - the Cavs were the biggest threat to our title run last season, and it stood to reason that with another year of Lebron developing, they'd only be better this season. The general public didn't jump on it, but wily NBA observers, such as Simmons and many of us here, would/should/could have. The Bird, you said yourself you saw them as the second contender for the title going into the season. Again, looking at the numbers above, it would have been rational for you to put money on Cleveland - far more rational than to put money on the Celtics (with your numbers, you'd expect to lose $6.25 betting on Boston but gain $110 betting on Cleveland).
As to Simmons' belief that he should be permitted to sell his ticket, it could never happen and under our laws should never happen. Gambling is only legal in certain locations like Vegas, so the sale through ebay or another provider of these options would be illegal if it was sold from a location where gambling is illegal or to a location where gambling is illegal. There's no real distinction between that and taking bets online. (i.e., I will sell for $100 a contract that says you will receive $200 if the Cavs win the title - that's just taking a 2:1 bet). It's just illegal.
Plus, Vegas would step in to prevent it. Vegas has no incentive to let people sell their bets. They want people paying them for 2-1 odds on the Cavs, since they get a commission on that. Simmons could sell a $300 15-1 ticket from the Cavs for $2000, the person buying gets 9:4 odds (better than the 2:1 available in Vegas) and doesn't pay a commission (as I recall, about 5%, so $100 on that Cavs bet in Vegas). That doesn't help Vegas. They want the commissions, even if they pay out more in the long run. They also want that extra money coming to them if the Cavs DON'T win so they can pay out on all the bets still coming in for teams like the Celtics and Lakers. In fact, if people were permitted to sell those Cavs tickets, Vegas would have to lower Cleveland's odds to entice more people to put money on the Cavs. They don't want to do that.