The Met Office's decisions have all been perfectly cogent, it's only the poor reporting that's led to confusion from lay people.
In the first case there were all signs of an eruption under the glacier. They issued an alert. Later there were no signs on the surface, so they removed it. Later on, glacial subsidence proved that an eruption had indeed taken place, but stopped. In both cases, correct behavior on their part.
Then there was the 1st Holuhraun eruption. When an eruption begins, theres no way to know how its going to evolve, but since it was just a lava eruption, it was only restricted on instrument-only flight and only to 5000 feet. When it died down, they removed it. Again, right call by the Met Office.
Then there was the 2nd Holuhraun eruption. Again, 5000 foot instrument-only restriction, and when it steadied out, they removed the restriction (yes, the Slashdot article is wrong, the restriction has long been removed). Again, right call by the Met Office.
People need to stop armchair quarterbacking, they're doing the right thing.