I have 15K reputation in one of the SE sites, I don't have such restrictions.
Now I entered a new SE site and I can't post twice in 20 minutes.
I think if a user is trusted somewhere he should be trusted everywhere maybe?
What do you think?
No, I don't think that if a user is trusted somewhere they should be trusted everywhere. If this would be true the same would go for reputation and privileges. Notice that you're only throttled because you're below the 125 reputation limit to prevent (spam) abuse as explained here.
In a new community it takes some time to get used to the specific rules for that site. You can't blindly assume that being good at Unix makes you a great member for parenting (or maybe I should have reversed those two but you get the point).
It will help though that you are an established user somewhere else on the SE-network and it won't take much to lift the restrictions given the fact that you are able to follow the rules and are familiar with SE philosophy.
And afterall getting 125 reputation shouldn't be that hard, on any site, specially if you take into account the 100 reputation association bonus you get. It only requires 12 approved suggested edits to get at 125.