From what I learned so far, I know that I should use "those(or that)" when I refer to something previously stated.
For example:
1. There are a lot of angry employees in front of ...
THOSE people are in fact....
2. Martin decided to join... In my opinion, THAT's a very good idea... (the fact that Martin decided to join...)

However, I've seen many similar cases in which "this"and "these" have been used instead of "that"and those". Why? Probably there's somthing I don't understand. Is there any difference between AE and BE usage? Or perhaps the rule is not very rigid...?

Hi Marco,

this v that & these v those

v - versus

this is near, that is far
these are near, those are far

This problem is easy compared to that problem we studied yesterday.

These animals we see here at the zoo look like those animals we saw in the wild last year.

Hope that helps.

In my opinion, that's a good idea. In my opinion, this is a good idea.

Both are correct. You're right; the rule is not very rigid. There are many ways to refer back to a previous idea. It's a matter of personal style. Most people, perhaps 95%, would say "that". Very few people choose "this" for the general case. However, if the speaker feels especially connected to the idea or the idea is somehow very important to the current state of affairs, "this" may be used to show the closer connection. (I don't think this reflects any difference between AmEng and BrEng.)
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Hi CalifJim,

Here's what GuruNet has to say.

USAGE NOTE This and that are both used as demonstrative pronouns to refer to a thought expressed earlier: The letter was unopened; that (or this) in itself casts doubt on the inspector's theory. That is sometimes viewed as the better choice in referring to what has gone before (as in the preceding example). When the referent is yet to be mentioned, only this is used: This (not that) is what bothers me: we have no time to consider late applications.•This is often used in speech and informal writing as an emphatic substitute for the indefinite article to refer to a specific thing or person: You should talk to this friend of mine at the Department of Motor Vehicles. I have this terrible feeling that I forgot to turn off the gas. It is best to avoid this substitution in formal writing except when a conversational tone is desired. See Usage Notes at that.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2003 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

That answer provided above supports what you just wrote.

I had based my earlier answer above on a Spanish text that discussed "este" (these) and "ésos" (those) when it referred to near and far as being the differentiating factor. I simply translated that over to English, which might have been in error.

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esto & eso are good parallels to this and that, but the Spanish-English correspondence gets even more complicated than that. Spanish has a three-way division, equivalent to the three grammatical persons, somewhat like "hither, thither, and yon". este - near me; ese - near you; aquel - near someone else (neither me nor you). Fortunately for the Spanish speaker learning English, the Spanish to English correspondence is easier to learn than the English to Spanish. At least I think so.

Thanks for finding that reference, too.