New York state voted yesterday, and both Clinton and Trump had resounding victories. As always, those on the losing side complained of "dirty tricks"
and "voter suppression". What happened is what is known as "the party system", which is what's supposed to happen during the primaries
. That is the function of the primaries, to allow the members of the parties to pick their candidate for the general election.
For better or worse, election law (other than campaign finance regulations) are left up to the states, so every state has it's own rules. Within the states, counties (or precincts) may have their own way of doing things. It's up to the campaigns to ensure that they know what those regulations are and comply with them, and ensure their supporters are doing the same. This is why it's advantageous for a candidate to belong to an established party - there is an infrastructure in place to make sure the "i's" are dotted and the "t's" are crossed - in every state, city, etc. It is difficult for new or independent candidates to run without this infrastructure, which is likely why Sanders decided to run as a Democrat, rather than try to create his own party.
Sanders as an "independent" (running as a Democrat) has electrified a certain sliver of the electorate that eschew the standards of the party system, and instead are chasing a ground-up reformation, where the old party system is no longer part of the equation. The realities of that hit them hard yesterday, when they found out that New York has a "closed" primary. This prevents members of one party (and - yes, independents) from voting for (or against) candidates in another party's primary. They also have rules in place to prevent members of a party from switching very close to the primary election. These are not new laws.
Closed primaries have their supporters and naysayers for various reasons, but it is the law in the state, and it is up to the voters to know the laws in their state. I - for one - support closed primaries. To me, an open primary is akin to letting workers in a non-union shop vote for the head of the union in the shop next door. The party is in place to bring a candidate to the general election - it is not there to cater to voters who aren't interested in the party, other than to vote for one candidate or against the other.
A lot of Sanders supporters are young, first-time voters who seem to be more concerned with supporting their candidate than doing what is required to actually get him elected - like registering to vote (and as a member of the Democratic party). Not allowing someone to vote in the Democratic party when they aren't registered to vote as one isn't voter suppression - it's the law.
I have read that persons registered as Democrats who did not vote for two years were switched to independent without their knowledge - preventing from voting in the primary. If so, that is a lousy rule. It does not, however, remove the onus from the voter to ensure their registration is correct. It has also been said that Clinton sent emails to her supporters reminding them to check their registration. Sanders did not. That is not evidence of Clinton "cheating" - it's evidence that her campaign is on top of what the situation and laws are in New York, and the Sanders campaign dropped the ball.
I have also seen reports that voting locations were changed suddenly. That does not help or hurt any candidate in particular - it hurts everyone. Every state has its own set of voting peculiarities - engaged voters and campaigns just need to be vigilant and work within them. A nationwide system that is simple and works for all voters would be ideal, but that would pretty much require a constitutional amendment, so for now - we all have to play with the cards we've been dealt.
The Sanders campaign (and - to a similar extent Trump on the Republican side) has struck a chord with voters tired of "business as usual" in politics. However much they hate it, though, they must play this game on the field provided and with the rules in place before they can achieve the power required to change them.