REVISED: This is a grammar question, but it is a question for generative grammar because it requires the Determiner Phrase distinction of X-bar theory in order to understand what is really different about each of the sentences. There is a noun-modifier in each of the phrases. The word order determines which word is the determiner+noun-modifier and which one is the noun-phrase noun. To the extent that one or the other succeeds or fails is a function of which makes a more sensible determiner for the other.
DP: The user The "johnsmith"
NP: "johnsmith" user
VP: "has been registered."
DP: [hey] ;)
DP: the tab the "userprofile"
NP: "userprofile." tab.
In the first case "the johnsmith" does not function as a good determiner for user. The "johnsmith" transaction history. The "johnsmith" hairstyle. The "johnsmith" Presidential Library. ok not that one. But "the user" is an excellent determiner: The user name. The user profile. The user agreement. The user him/herself. Etc.
In the second case "the tab" is a not a good determiner for "userprofile." At all. Except by counterexample: "The tab layout" "The tab feature" It is for introducing things that tabs have. Not the other way around. Whereas "the userprofile" is an excellent determiner in this context: "the userprofile page" "the userprofile settings" "the userprofile object/property/servlet."
This is not a grammar question. Both of the sentences are perfectly grammatical. But they have different meanings. So the reason that one sentence sounds correct and the other does not is that the subtle difference in meaning leads to one sentence which comports with your understanding of what has happened and another which does not.
The user "John Smith" has been registered.
The "John Smith" user has been registered.
Their grammar and thus their meaning is different which I know immediately upon reading them as you do. I can only explain why if I try to translate them into Latin or Russian where there is full suite of case markers.
The user is subject in both sentences. Subject = Nominative. Easy.
But the case marker for "John Smith" is not the same for both of these sentences. In the first sentence, In Russian, "John Smith" stays in the Nominative case. The one and only user "John Smith" is the subject of the sentence. In Latin, I want to use the Vocative. Which is still just subject case, used for direct address. That may not be correct. It might just be nominative. I was often accused of over using the Vocative case. (Which exists in English only poetically as the particle "O.." "O judgment, thou art fled to brutish beasts." Judgement is clearly the subject of that sentence but is also being spoken to, sort of. The user "John Smith" has been registered. juror number 2 has been released. I understood this form of specifying by name as the criteria and application for Vocative. Or just Nominative, simple subject case
In the second sentence, "John Smith" would be in the genitive case. Because "John Smith" is the type of user or a quality of the user. The user belongs to the class of users known as the "John Smith" users. The user is an instance, just one example of a "John Smith" user. Well that is not right at all.
The user has been registered. Is a simply declarative sentence which is just a little too vague. Which user? Why, the user "John Smith" if you must know.
A user has been registered. A user? What kind of user? A "John Smith" user of course. Which user is it? The "John Smith" user (?!) The only reason The "John Smith" user seems like it unambiguous is because we have separate understanding of "John Smith" as the name of one person. But user names can be anything.
The user "blue toothpaste" has been registered. The specific user with the name "blue toothpaste" has been registered. This is the meaning you intend.
The "blue toothpaste" user has been registered. The user of the so-called blue toothpaste has been registered. Oh no not another blue toothpaste user. This is not the meaning you intend.
The juror number 1 has been released. == The specific juror with the number 1 has been released.
The number 1 juror has been released. == The best juror has been released, Juror 6.