Boston Approach Fussing

During a receint trip from Washington, DC to Whitefield (HIE) in late June of this year I had a problem with one of the controllers from Boston Center. I was at 9,000 feet IFR and IMC proceeding to the Montpelier (MPV) VOR and was planning the 044 degree transition for the LOC/NDB RWY 10 Approach. HIE is a tricky approach usually requiring an intercept of the MAHN NDB with a spiraling hold decent to final approach altitude of 4,000 feet. Beyond the final approach fix the LOC/NDB RWY 10 transition is quite steep and you have to be quick on the altitude transition or you will find yourself too high or exceeding your target final approach speed. There are mountains left, right and just ahead after the airport relative to the approach so you really have to be on your needles. Boston Center usually brings you in at 8,000 or 9,000 feet from the south to clear the mountains (6,700 feet) directly to the MAHN NDB. From that point you are on your own navigation for the approach. The Montpelier transition goes through a valley. The transition altitude is 4,900 feet. Its a good transition especially in IMC because at 4,900 feet you still have to do a proceedure turn but you only have to decend 900 feet to the final approach fix. When I was 5 miles from the Montpelier VOR Boston Center cleared me for the approach (no altittude restriction was given). Once passing the VOR I immediatly deceded to 4,900 feet as depicted on the approach plate. Once reaching 4,900 feet Boston Center scolded me for departing my assinged altitude. They immediatly requested me climb to 7,000 feet. I proceeded to the MAHN NDB and completed the approach. My question is that why did the controller have a problem with my decent? I understand that once "cleared" for the approach you follow the transition to the approach unless ATC gives you instructions otherwise. Was I wrong or was the controller in error?