First this is the whole rule, we are referring to :

We typically allow all giveaways to run their course without judgement, and allow the giveaway creator a fair chance to prove themselves and to come through with the gift in the end. An exception is when the giveaway creator enters a description that clearly states the giveaway is fake, or if the description illustrates a lack of understanding of how the giveaway system works. Please refrain from posting any comments accusing the giveaway creator of posting a fake giveaway, and only create a support ticket if the giveaway is fake beyond a reasonable doubt.

It is rude because you are not allowing the

giveaway creator a fair chance to prove themselves and to come through with the gift in the end

instead you are implying it could be fake.

Furthermore: saying that it "could be a fake giveaway" is still infringing this rule as there is no evidence that the giveaway creator

illustrates a lack of understanding of how the giveaway system works.

I understand that you just want to "warn" people from entering giveaways that look suspicious to you, but it's just not necessary.
I personally think that rules like these are important as they support a behaviour that gives new people a chance to prove themselves without having to deal with prejudices.
If you spot a giveaway that looks suspicious to you, then you can just nicely ask the creator if he fully understood how the site works* or you can just ignore it.

People, who joined a giveaway that turned out to be "fake", lose basically nothing. They just spent virtual entry-coins, that they received from this site for free. This might be grave for some, but in my opinion it's a small price for an all-in-all better community*-environment, that is created by such rules.