I’m easily angered, but this is one of those things that really, really, **really ** chaps my ass, and I do everything in my power to see that businesses change their policies or pay the price. Sadly, when I have to go through my actual bank, they often act like they’ve never heard of the merchant agreement.
“Wait, they tried to set a minimum purchase amount? Huh? Is that bad?”
Luckily, Mastercard has that one form that you can use right on the website to report merchants that pull this crap.
It was a major culture shock when I moved to Chicago from the east coast, where I was used to never carrying cash and using my card for everything. It’s a real midwestern trend to set minimum limits, and I’m fighting them one at a time.
One that really burned me up recently was a business that had a freaking engraved plaque on the wall proudly declaring their $10 minimum transaction for using Visa. The audacity of these folks! The idea of them going out of their way to custom order a plaque flaunting the “rule” just blew my mind, and I asked to speak to the manager. I explained to him the merchant agreement to him and advised him to do away with it, or to talk to someone (the owner?) that could. He basically told me that there was nothing he could do, so I reported them through my bank and through the Mastercard website. Next time I went in there, the plaque was gone. 
Something I’ve found that really helps send a message (in ANY situation in life, but especially in a situation like this) is to keep a notepad and pen with you at all times, and to whip it out whenever there’s trouble and you need to hammer home just how serious you are about it.
[scribbling furiously while talking to manager] “So, one last time, you’re refusing my visa card because it doesn’t meet a minimum limit that you’ve set, right? [scribble, scribble] And even though I’ve informed you of the merchant agreement, you still won’t accept it. [scribble scribble] Ok, and the address here is…[looks around, then scribbles some more] And once again, your name is Bob, correct?[scribble scribble] and you’re the manager on duty… [scribble] Bob, what’s the phone number here?”
They usually panic at this point and allow the transaction. I still report them, though, for the next guy that comes along.
And anyone who disagrees with this, it’s this simple: don’t put that Visa sticker in the window and then refuse my transaction for any reason. If I see that sticker in the window, it means I can use my card to pay there, period. Not for a minimum amount, not if I show you my I.D., PERIOD.