In this case, Maple Leaf is a proper noun which are not bound by the same pluralization rules that apply to common nouns.
You may have wondered why it’s the Toronto Maple Leafs, not Toronto Maple Leaves. It’s because proper nouns have different properties than common nouns.
For example, if your family name was Leaf, you would be collectively described as the Leafs, not the Leaves. Even though those same Leafs might rake up leaves in the yard.
An email to the University of Toronto department of linguistics cleared up this confusion (if there was any). J.K. Chambers, a professor in the department, graciously answered the “Leafs vs. Leaves?” question the day after the Capitals and Maple Leafs started their first-round playoff series.
Bonus Biscuit – Which came first the Toronto Maple Leafs or the Canadian Maple Leaf flag?
Why then, did the team get named the Maple Leafs?
The [Leafs] link to the military extends back nearly 90 years, when then-Leafs manager and hockey icon Conn Smythe decided to change Toronto’s NHL team’s name from the St. Patricks late in the 1926-27 season. He chose the name Maple Leafs because the grand majority of Canadian military regiments in World War I wore a maple leaf badge – and it was not a token gesture on his behalf.