The real problem is the computer program they implemented last year at all border points. It's set to flag visa exempt entry stamps after 4 or 5 entries.
In their "finite wisdom" the people who designed the program didn't bother to put in a way to track days between visa exempt entries. It flags you whether you've done four back-2-back visa exempts or only entered on a visa exempt entry four times in a whole year. I give them credit for tryin' to stop people from living here on back-2-back visa exempt entry stamps, BUT they could have designed it to target that demographic NOT everyone.
Also, while the immigration officers at passport control can "tell" or "strongly suggest" you get a proper visa, what they can't do is deny you entry on a visa exempt stamp, especially if you meet those "3-proof-of's".
Don't take the word of some half-wit, low level passport stamper, push it up the ladder. At this time there is NO rule that limits the number of visa exempt entries you can get in xxx amount of time entering by air, period.