After reading the whole thread, I had a couple of comments: first, NOBODY works harder, works longer hours, takes on more thankless jobs, and is more dedicated than my BM - and I honestly believe that most! BMs (and BOs) have this same work ethic - or close to it.
They are sacrificing their free time, their labor, their very life to look after the horses in their care - for very little compensation. Some are better caretakers than others, obviously!, but their profit margin is almost always slim. TOO slim, IMO, for the amount of backbreaking work required.
My BM has a huge operation: 500 acres, 150+ horses, broodmares, babies, layups, retirees, and tons of youngsters in various stages - with a handful of boarders who ride (me being one of the very few.) My board is inexpensive due to the fact that there are sooo many horses there - and my two (one on stall board, one retired and on field board), are very low-maintenance, as am I.
There are always compromises, of course - but my horses are happy - and that’s the most important thing for me.
Anyway, my other point was that as I read the OP’s OP, she wasn’t “quibbling” about the board increase, per se, she was puzzled by the significantly larger percentage increase for the field boarders. It sounds like the pasture board is “minimal care” - much like mine is. In my case, my horse gets a field (a decently sized one, with okay grass - though she has recently been moved to a smaller T/O with more horses around her), a water trough (no heater - they break the ice once a day in winter), no shelter (I have great T/O blankets), no grain - a few times I have asked that my horse gets a small amount but this has been a total of 6 months at various times over a 10 year period), and roundbales in winter. My horse’s T/O field gets mowed a few times a year, never dragged (and no manure removal); the weeds do get bad - but again, the fields are large so it’s less of an issue. They do replace fenceboards, but it’s on a leisurely schedule (so sometimes days to a week or two if there’s a broken board.) There is a room in a nearby barn to store blankets and supplies (and I could bring my mare into the barn) but no ring - she’s retired, like the rest of the horses in surrounding T/O so no real need. My BO does put on and remove blankets as needed (or change them), though this is maybe? 8-10 times a year. I try to ask her for next to nothing, and for no extras; she has enough on her plate. I leave fly spray by my horse’s field and if a worker has spare time and is filling a trough nearby, they will spray her. (15 times a year or so?) There is an eye on her once a day. VERY basic care. Luckily she is extremely hardy and an excellent keeper with great feet - just a healthy horse - I’m very lucky! 
If the OP’s situation is similar to mine, it seems surprising that her field board went up this much unless there was some new associated cost?, and of course board should always be raised to cover increased costs so the BM isn’t subsidizing horses, but actually making some kind of a VERY much deserved profit.
I think that’s what it boils down to.
In which case it’s not at all unreasonably to inquire as to why it wasn’t “in line with” the increase for the stall boarders (expecting that board increases are normal and to be expected); it’s likely there is a reason (so not unreasonable to ask), and the OP seems gunshy as a result of past experiences so that is probably why she was concerned. Obviously BMs need to turn a profit, and I don’t think she was suggesting otherwise!
For those reasons, I think the jumping on the OP is a bit excessive. Just my $0.2.
