The direct answer to your question would be because the designer(s) of the TAHITI program decided to reprogram the TAHITI patients do so.
But that's not a real answer, as far as the core of your question.
TL;DR
It's much easier to seem truthful when you are telling a believable lie; than it is to seem truthful when you refuse to answer the question (or are incapable of doing so).
This "magical place" recorded response reminds me of a similar programmed behavior.
In The Wolf Among Us (videogame), you play as a detective trying to uncover a secret plot. Something has happened in the past; and anyone who was witness to it has been cursed (i.e. magically rewired).
Whenever those witnesses try to speak about the events that occurred; they are only capable of saying "these lips are sealed". When discussing unrelated topics, they have no such limitations. It only happens when that specific past event is the topic of discussion.
The above example uses magic, whereas project TAHITI is grounded more in fringe science. But the intention of wanting to keep something a secret is the same.
There are a few things to take note of about Coulson's memories:
- The designers of the TAHITI program want their patients to be unable to remember their experiences and talk about them. It needs to remain a secret.
- Coulson's brain still contains the memories of what happened in TAHITI.
- If Coulson acknowledges that something secretive happened in TAHITI, and he focuses on trying to remember it long enough, he is capable of recalling certain memories (in bits and pieces). They never deleted Coulson's memories; they simply buried them deep in his subconscious.
Why would you make the patients say "it's a magical place", rather than making them say nothing at all?
Although it is never directly proven; I think I understand why it's better to have them say a "recorded" response rather than remain silent.
- If anyone asks Coulson about TAHITI, and Coulson does not answer; that warrants further investigation. Especially if Coulson completely shuts down whenever the topic is brought up.
- If anyone asks Coulson about TAHITI, and Coulson gives them an empty sounding phrase; this will seem much less suspicious. It sounds like he is redirecting the topic of conversation, simply because he does not want to talk about it. Most people would take this hint and not press Coulson for details, as a matter of politeness.
It's much easier to seem truthful when you are telling a believable lie; than it is to seem truthful when you refuse to answer the question.
In this case, "it's a magical place" isn't really a lie. It is an implication that Coulson is talking about Tahiti (the island), rather than TAHITI (the secret program).
And there's a second layer to it. If you don't believe that it's about Tahiti (the island); it still seems likely that Coulson has intentionally avoided the topic simply because he does not wish to talk about it.
No one realizes that Coulson himself doesn't remember it; because he talks about Tahiti (and how it's a magical place).
But it serves the same purpose: it lowers suspicions.
- Someone talks to Coulson, asking how he is still alive (the TAHITI designers would expect Coulson to be asked this question a lot of times)
- Coulson gives them a false reply, making it seem like he's talking about Tahiti. This makes Coulson's "trip" less suspicious. Even if they do not believe it's about Tahiti, that person would still infer that Coulson is not speaking about it in detail because he does not want to talk about it.
- Because suspicions are lowered, it is much less likely for this other person to keep interrogating Coulson about it.
- Because no one is interrogating Coulson about it, Coulson will spend less time thinking about TAHITI
- Because Coulson spends less time thinking about TAHITI, he will not be able to recall these memories that have been buried deep in his subconscious.
