Does Unity Store allow to use assets listed as free in games built without the use of Unity engine? It's a pretty big place for free assets and I wonder if this isn't against the Unity Store's usage agreement.
The information you seek is readily available through the Asset Store ToS and EULA. To me, it infers that you are free to do so. The only catch appears to be that you must download the assets through Unity, and that you are purchasing a license for the asset.
All that aside, I am not a lawyer. To make matters more difficult, you can be sued for anything. For example, you might be legally allowed to use the assets in a competitors engine. That does not mean the Unity team (or any other involved party) can not sue you. It does not even mean that they will lose their case, if they sue you. If the case still makes it to court, it is still up to the judging party (often a jury, but again, this may not always be the case) to decide. In some cases, they may decide that pre-existing legal documentation such as a EULA is overridden, due to other unforeseen issues such as common use and precedent.
All legal mambo-jumbo aside, I took the liberty of asking a Unity representative through contacts they provide on the website. Having been asked if any asset could be used in a competing engine, or even in a custom home-brewed engine, the reply was "In short yes this is allowed".
Note that it is common for all assets to be referred to as "purchased" once the user acknowledges acquisition, be it through a checkout system or through initiating the download. It is also not uncommon for users to have to "purchase" free assets, although the price should still be listed as $0. Reasons include processing the assets through the same system, that still expects and processes a sale price (sometimes behind the scenes), and in order to issue a receipt of the transaction (which is just as valuable if you ever need to prove that Unity provided these assets to you for free, in the first place).
As per request, I replied to Alex and asked if this would include the free assets. His reply confirms that yes, it does include assets you do not need to actually purchase.
In regards to your query, yes this is correct, all assets downloaded will go into the directories previously mentioned.
Interesting question, as this impacts freedom of developers creating games in Unity that may need to move to a different framework over the time (and thus take some of the free or purchased assets outside Unity).
I'm not a lawer. I have read through ASSET STORE TERMS OF SERVICE AND EULA (all caps from the original).
What I take from there, regarding your question, is that you shall check each Asset particular license, especially as some may include Open Source content which license must prevail. However, in general terms, Unity asks all publishers to agree to a common license, and it does not limit the content to Unity (as of September 2016):
Licensor grants to the END-USER a non-exclusive, worldwide, and perpetual license to the Asset to integrate Assets only as incorporated and embedded components of electronic games and interactive media and distribute such electronic game and interactive media. END-USER may otherwise not reproduce, distribute, sublicense, rent, lease or lend the Assets.
There are other relevant parts you shall check, for example "You may not access Unity Store in any way except via Unity".
Also make sure you check these related threads:
I got an email response from a customer service engineer at Unity's Zendesk:
You can purchase assets from the Asset Store, or download our free assets, and use them personally and commercially in your game.
You may use the majority of assets from the Unity Store in other engines without limitation. Please take some time to look at our terms documentation for further details on limitations of usage. This is found here: https://unity3d.com/legal/as_terms
Once you have purchased an asset from the store, it becomes yours to do with as you like within your games and apps. You will be able to use them in your game for commercial use with no extra payments.
You can also use free assets in your game for commercial use. Using Unity assets is fine, though please be aware that many developers use Unity to create their games and most are aware of the assets we provide. For an original game they might not be the best fit.
You cannot resell the asset, or include it in an asset type product.
This answers my question about free assets specifically.