I am not an expert on this, so I'm not saying you are wrong, but why would they not have the prerogative to do this? Do you have any sources for that?
I know at least that there is precedent in the EU - Austria cancelled a round of elections in 2016, also for some electoral law incongruities / technicalities (that at least superficially by my knowledge were less serious than this scandal), done also by their constitutional court.
What they found afterwards, on their own without any case brought before them, is that the campaign that preceded the first round of elections (which lasted for one month before the first round two weeks ago now) may have been influenced by outside forces, that a candidate may have flaunted campaign finance laws, and similar matters, and that because of this, the entire electoral process is invalid and annulled. The government has to restart this process from scratch, with anyone who wants to participate registering their candidacy again from scratch.
There is no procedure or standard in any law or in the Construction to specify such a process. The Court invented it from whole cloth, based only on a vague/broad power to oversee the election.
I am curious to read more about this in the coming days. I do remember previous scandals related to the CCR and their (often said too close) relationship to the parties in power. But I guess in this case I just don't see why their decision would be illegal, and when compared to the alternatives I don't see why it would be wrong.
To also clarify, despite thinking this might be the correct decision, I actually think politically there is a higher probability now of yielding a worse president, since Ciolacu & Simion will probably end up in the secondary, the latter having chances, instead of Lasconi. Not that I think she's the greatest candidate either, but again... compared to the alternatives...
It is precisely because of how that power works that what they did yesterday is illegal, in my opinion.
The Court is not basing its decision on whether to validate or invalidate an election result on the Constitution directly, in regular elections. The constitution is too broad and vague for this kind of power. Instead, based on the constition, specific laws that govern how elections are run and at what parts of the process the Court is involved in them were elaborated, and those laws were validated by the Court itself. Specifically, this law is 370/2004 [0].
If the Court can just convene itself by fiat, analyze any evidence it wants, and decide what effect that can have on the election, then why did we need a specific law with specific articles on how election results are validated in the first place?
[0] https://www.roaep.ro/legislatie/wp-content/uploads/2019/08/L...