|Image from Huffington Post|
For starters, Bernie Sanders has long spoken out against our two-party system, both because of the corruption found there and because of how our third-party intolerance restricts our choices for president. Some people have found it hypocritical of Sanders to run on the Democratic ticket after declaring himself Independent for so long.
It's important to note that Sanders has not switched parties. He is still Independent. He is simply running on the Democratic ticket. It may be surprising to know the Democratic nominee for president doesn't have to be a Democrat so long as their "record of public service, accomplishment, public writings and/or public statements affirmatively demonstrates that he or she is faithful to the interests, welfare and success of the Democratic Party of the United States" (see previous link).
Also important to note is that there are laws in some states that can prevent an Independent from being listed on the primary ballot, specifically New York and New Hampshire. Since Sanders hasn't changed his party status there is no guarantee he'll be able to get around these laws, but running on the Democratic ticket improves his chances. (And it sounds like New York won't be a problem.)
Sanders has also said that he's not interested in splitting the Democratic vote. If he ran as Independent, he'd mostly draw votes from Democrats, splitting the Democratic vote between him and Hillary, giving a chance for the Republican party to take the White House. Why would he risk that?
Image from Daily KOS
Critics of Sanders call him hypocritical for aligning with a party he claims to disagree with. They say if he really wanted to lead a political revolution, he'd run as Independent and spur American citizens against both Democratic and Republican parties. Others, who see his campaign as hopeless, criticize Sanders for not setting his political revolution on more "realistic" sights where his calls for change could still flourish, such as running for governor.
Suffice to say, Sanders is drawing massive crowds with his speeches against inequality and big money corruption in America. And polls indicate much of America agrees with his views. It's looking more and more likely he's posing a problem for Hillary. If (when!) Sanders wins the Democratic nomination, he'll have one (and possibly his most serious) opponent down before the General Election (as opposed to having to face two opponents if he'd ran solely as Independent).
I don't expect right-wing media to stop bashing Sanders (it's kind of their job), and I imagine some citizens who ally themselves as Republican won't open their eyes either (although, have you seen the Republicans for Bernie Facebook group?). But I do believe there are people who simply don't understand the facts and well-formed strategy involved in Sanders running on the Democratic ticket.
As Sanders has said, he's in this to win and he should not be underestimated.