The speech act of promising has the following conditions:
S promises H to do something.
1. S believes H wants something done. So the hearer prefers to receive the act.
2. S is able to do A
3. S is willing/ intends to do A.
4. A has not already been done. So it will be accomplished in the future.
I believe the answer to your question lies in the blue part of the first condition: the hearer, in case of a threat, will not be willing to suffer the possibly dangerous act. The speaker should be capable of doing the act in the FUTURE.
The vocative sentences that are usually found in the legal statements are good examples of such an illocutionary act.
As a result, The following statement is not felicitous:
- I'll give two thousand dollars. This is a violation, because two thousand dollars will be something desirable and unthreatening.