BSD talks mostly about redistribution and doesn't even mention commercial use or something like "for any purpose". Therefore they don't need to explicitly state "commercial use is allowed" or "for any purpose" for others to use commercially freely right? Also I saw the Open Source Initiative say no open source license can restrict a party from doing so I'm confident that you are allowed, but just wondering why it's not stated.
Please explain why it's unnecessary thanks
The basic rationale of all licenses is that you are not allowed to copy or distribute someone else's works that is protected by copyright, unless you have their permission. The license states what you are permitted to do, under what conditions (and may also carry disclaimers which could legally protect the copyright owner in case the software causes damage). If you violate the terms of the license, you could be sued by the copyright owner.
Many licenses are partially unclear, in which case if a copyright owner decides to sue a person for infringement, they have to prove various things. The user can defend themselves by showing that they have permission; the copyright owner would then have to prove that the user violated the conditions of the license. The burden rests on the copyright owner to establish that the usage in question is not allowed under the license. The argument rests almost entirely on what the license says (except that certain clauses might be deemed illegal, and certain uses e.g. "fair use" might be deemed to be allowed regardless of the license terms). We can take this "three-clause BSD" to be an example of the license class that you are asking about. Omitting the disclaimer, the license says:
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
(1) Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
(2) Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
(3)The name of the author may not be used to endorse or promote products derived from this software without specific prior written permission.
The first sentence efffectively says, in the most general form, that the work can be copied and redistributed. Then there are conditions, none of which are about commercial use or gratis redistribution or "by any person in Nepal" or anything else. In order to impose a restriction, you have to actually say what that restriction is, and you can't rely on a belief that "of course this would not include {commercial use; translating from C to Java; use in a defense-industry program}". The wording is sparse, so it is indeed unclear whether they "intend" to also allow printing the source code and using it as decorative art. You could argue that "use" means "use, in any way you can imagine". A reasonable person would conclude that the copyright holder intended "use" to me "use in any way whatsover". The plain meaning of "use" is most strained in the case that a person simply copies the work but but not use it, the significance of that being that copyright law is centered around copying, not use. So the plaintiff would have to establish that a reasonable person would know that "use" means both "copy and do something with", and simply copying is not permitted by this license. That is such a stretch that I don't see any court finding that there was infringement for lack of "doing something with". The point is that the court will have to determine what is a reasonable interpretation of the words in the license.
