The symbol was not only the game-logo in the first game either. In fact it makes various appearances in Black Mesa, first and foremost on Gordon Freeman's HEV suit itself. But there's also the Lambda Complex that Gordon enters later on (from where he gets teleported to Xen), that employs this symbol as its logo, too.
If we then set this in relation to the somewhat legendary meaning that Black Mesa, and Gordon's adventures there, have for the whole ideology of the Resistance, it seems to be a reasonable choice, or at least within the bounds of plausibility. (Even if the most important reason would probably still be the out-of-universe one of a recurring trademark and symbol of identification for the player and the game in unison).
Those points are also reinforced by the Half-Life Wiki, but they're not beyond reasonable deduction in the first place:
In the original Half-Life, Gordon Freeman's trademark HEV Suit was marked with a Lambda Logo on the chest, as were other HEV Suits. A section of Black Mesa dedicated to teleport research was also called the "Lambda Complex," and is identifiable by the Lambda logo at its entrance...
Gordon's Lambda logo on his suit and the work of the Lambda Complex became something of a legendary trademark symbol for the Resistance against the Combine, referring to the acts of Gordon Freeman during the Black Mesa Incident.
