if you want to use something that others own/have build/created, ask for permission if license is not defined free. very easy rule simply out of educated good behavior that everyone should have learned since kindergarden.
1) "may i play with your toycars?" "sure, lets play together."
2) "i play with your toycars now!" "no, you dont" bamm! "uääähhh."
when you are grown up playing together means licensing, while "bamm!" means dial the lawyers number.
thinking of that we are all creators want to be respected, and of the great Immanuel Kant:
"Was du nicht willst, das man dir tu, das füge keinem anderen zu."
"Do not do to others what you do not want others to do to you."