Signing up for something isn't instantaneous. It's not just signing a contract - it's a case of going somewhere, talking to people, doing paperwork - possibly queuing up before being able to do those things, or between stages. If it's not done in person, it still might have several stages.
The present perfect indicates that it is known that the action is fully completed, and the duration of the activity isn't important, and that you're not trying to refer to a specific point in time, or period of time, during which the activity of signing up was happening. Also, just because the present perfect is acceptable doesn't mean it must be used; you can use the present perfect progressive as well, and it changes the nuance. For example, it can create a greater impression of immediacy - that the signing up happened more recently.
Both are acceptable, with difference in nuance and some contexts will affect which is 'right' (and it could be both). The justification that it is a temporary activity for saying that only signing is acceptable is utterly spurious. A temporary activity that has been completed can use the present perfect.