The Problem of the Rifle

I just re-read the poem "The Highwayman"  for the first time in years, helping my sophomore daughter with it. I enjoy the poem, but I found myself wondering--exactly what is the deal with the rifle?

They had bound her up at attention, with many a sniggering jest!
They had tied a rifle beside her, with the barrel beneath her breast!
"Now keep good watch!" and they kissed her. She heard the dead man say, 

Why specifically did the soldiers tie it beneath her breast? It is certainly instrumental to the plot that it be there:

The tip of one finger touched it! The trigger at least was hers! 
The tip of one finger touched it, she strove no more for the rest; Up, she stood up at attention, with the barrel beneath her breast. She would not risk their hearing, she would not strive again, For the road lay bare in the moonlight, Blank and bare in the moonlight, And the blood in her veins, in the moonlight, throbbed to her love's refrain. 

Tlot tlot, tlot tlot! Had they heard it? The horse-hooves, ringing clear; Tlot tlot, tlot tlot, in the distance! Were they deaf that they did not hear? Down the ribbon of moonlight, over the brow of the hill, The highwayman came riding-- Riding--riding-- The redcoats looked to their priming! She stood up straight and still.  

Tlot tlot, in the frosty silence! Tlot tlot, in the echoing night! Nearer he came and nearer! Her face was like a light! Her eyes grew wide for a moment, she drew one last deep breath, Then her finger moved in the moonlight-- Her musket shattered the moonlight-- Shattered her breast in the moonlight and warned him--with her death. 

But it seems way too contrived. It just doesn't add work for me. The best theory I can come up with is that the soldiers thought The Highwayman would embrace her and cause the rifle to discharge. But that seems less than half-baked. Am I missing something? Because I hate to be a spoil-sport[1], but to base such a central plot element on such an illogical circumstance just undermines the whole enterprise.

(I think Noyes could have achieved the same plot effect with more plausible details...she could have tried to wrest a gun away from one of the redcoats, and been shot in the struggle.)


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[1] Okay, those who know me will realize that I don't actually hate being a spoil-sport in questioning things this way, it is probably more fair to say I thrive on it...