It's a series of choices, but those choices can be misguided or wrong without maintaining communication with the group. It also easily slips into the "white savior" trope.

Imagine a person calling themselves a queer ally while actively protesting "LGB without the T". They are absolutely NOT an ally to the queer community, but without checking in with queer people, they wouldn't know. Queer people then have to argue with "an ally" to get them to understand why that rhetoric is harmful, which just puts everyone further back.

So, people can call themselves allies, but no one should believe it unless it's been attributed to them by the group they're allied with.