That isn't what they've done at all. They've taken control of a company using the assets of that company as collateral, completely stifling it with debt.
It's like me taking over your corner shop because it's full of stock, flogging all the stock to pay for it, then replacing all the brands with cheap knock offs and charging you 50% more than you were paying previously because the shop is now loaded with debt.
The reason FC United (and the anti-Glazer campaigns) were created was because fans objected to directly paying for some US billionaires to own their club.
They took over Man U in 2003, since then the Premier League and Premier League clubs have seen a massive increase in support outside of the UK and increased TV deals within the UK. Football finances have gone through the roof, and Manchester United have gone backward at almost each and every turn. Imagine saying in 2003 that Manchester City would surpass them!
Don't get me wrong, I have no love at all for Man United and seeing their decline has been hugely enjoyable, but the thought of the Glazer's lecturing anyone on how to run a football club is laughable.