Try not to think of this as the future simple being used for habits. The modal will has several uses, one of which is to express futurity.
1. The host city of the 2016 Olympic games will be Rio de Janeiro.
2. John will talk for hours about his garden once he's started.
In #1, the speaker is indicating a future certainty. Some people call this use of will the 'future simple'In #2, the speaker is indicating a characteristic of John. There is no way in which we can regard this use of will as an indication of futurity.
2. John will talk for hours about his garden once he's started.
3. John always talks about his garden when we have guests.
4. John is always talking about his garden.
In all three of theses sentences, the speaker is indicating that John often talks about his garden. The difference in meaning is not necessarily significant, but there is a difference.
In #2, the use of will suggests that this is a characteristic of John. It may or may not be an annoying characteristic. Only context can tell us that.
in #3, the speaker is simply telling us about something that happens regularly.
In #4, the use of always, normally associated by virtue of its meaning with the unmarked tense seems at first sight illogical. However, the use of the progressive aspect with an action can stress the repetition of that action. The combination of the progressive aspct and always tells us that this is a situation that actualises repeatedly, but because the duration of the whole series of talking is limited, it is not presented as a permanent state of affairs
This combination is associated by some writers with some idea of the speaker's emotional attitude, but this will be made explicit not just by the words, but by the whole context of situation and the speaker's tone. It is not true to suggest that it always expresses the speaker's irritation; with change of tone of voice and facial expression, the person uttering #4could express irritation, resignation, amusement or a number of other feelings. Here, as is almost always the case in English, it is context and other factors that express feelings, not simply the words. The combination can just as easily be used to express pleasure, as in:
5. He's always buying me flowers.
Here's another use of will that is not really a 'future simple':
6. Our guest speaker can't make it this evening. Don't worry; John will talk about his garden.
Here, the talking is in the future, but the speaker is mainly indicating John's willingness to talk.