This main thrust of this question was recently addressed here by myself and others, as well as in some older posts like this one.

A key notion to keep close to mind here is that concepts of loyalty must be viewed in context rather than as polar absolutes. To wit, the thirty years between the 1715 and 1745 risings saw significant shifts in how Britons felt both about the established government and about Jacobitism as an effective alternative. That's a full generation of belief, statecraft, economic change, memory, and personal fortune, all of which rarely remain static. The same span of time between Culloden and Lexington/Concord likewise provided enough distance for all of these states to evolve, and for supporters on both sides to evolve with them.

Particularly pertinent to the phrasing of your question, it is not accurate to claim that the Macleans were 'fervent' Jacobites all through the eighteenth century, mostly because clans never adhered or committed to Jacobitism as a discrete unit with cleanly defined margins. Allan Macinnes shows in his chart of major Highland clans that three septs of Macleans are considered amongst the fifty largest clan-based groups of the era, and that two of the three were generally Jacobite-inclined in 1715 but were largely neutral by 1745. [Clanship, Commerce and the House of Stuart, 1603-1788, pp. 247-9] Surnames alone didn't mean a whole lot in the eighteenth century and the idea that particular clans were wholly loyal Jacobites or staunchly opposed to the Stuarts through a century of civil war and international political intrigue is simply myth.

In the same sense, how Scotland and the Highlands, in particular, were viewed and treated within the context of the British empire was rapidly changing through the eighteenth century, and it sped up hugely after Culloden. To summarize the situation in very few words, after the legal restrictions and horrid depredations inflicted upon recalcitrant communities not only in the Highlands in the 1740s, the same martial spirit was deliberately harnessed by the British military to serve on the front lines of its burgeoning empire. In this shift we see a sort of rehabilitation of Gaeldom, however mythologized this was even in the moment, as the template of loyal Britishness even just a few decades after Gaeldom's arguably lowest point at the close of the Forty-five. [See Robert Clyde's From Rebel to Hero: The Image of the Highlander 1745-1830 and Geoffrey Plank's Rebellion and Savagery: The Jacobite Rising of 1745 and the British Empire for more on this]

As a last note, we'll want to remember here that the Highlands were mythologized into Jacobitism, especially as the century wore on. The close majority of Jacobites in Charles Edward's army in 1745-6 were from north-eastern Lowland regions distinctly behind or bordering the Highland line. Jacobitism was never an affair of Gaeldom alone; however prominently Highlanders featured in propaganda, memorial legacy, and on the front lines of rebel regiments during the risings, it was always an international opposition movement set against the backdrop of European power politics. It therefore makes sense that thirty years later, some of those who once wanted a Stuart back on the British throne might have changed their minds as American upstarts made the first gambit to remove themselves from an empire of which some ex-Jacobites had grown to be fond. And for others still, it was simply their job to do so.

Hoping some of this has been helpful to you!

Yours,
Dr Darren S. Layne
Creator and Curator, The Jacobite Database of 1745