As thay say in user aggrement QA
:On my site there must be a registered user. Can I use the API Yandex.Maps?
Yes, the Agreement permits the use of API Yandex.Maps on sites to which access is limited enrollment. It is important that the registration was free. You should also make sure that your site does not violate other provisions of the Agreement.Can I use the API Yandex.Maps on a commercial site?
Yes, you can use the API Yandex.Maps on a commercial site, but only if this site is not inconsistent with a number of restrictions contained in the User Agreement.
In particular, your site should not charge for viewing information on maps, and the charge for the placement of users or advertisers of information on the map you can. For more detailed information read the User Agreement in the appropriate section.
ButCan I use the API Yandex.Maps in applications other than Internet services?
No, it is prohibited User Agreement.
Seems they check some header in api-requsts to determinate page which sends request. And checks the Key.
Also they don't check key for localhost and 127.0.0.1 (for developing purposes) and don't requre key in case of direct request (not from site page JS)
Seems request from users device will be prefered. And phone app which give some online functional can be called "inernet service" in contrast to your server requests.
So what do you think?