Walking into McDonalds is not maris ayin. Everyone knows you could be going in there to get a coke, go to the bathroom, etc.
If they don’t lock their wifi there is nothing illegal about using it. They could lock it if they didn’t want you to use it. Therefore there is no reason it should be assur with regard to dinei mamonos.
If the general perception of people is that what you are doing is wrong, even though it is not, then if you are noticeably Jewish/religious you should not do so because it would be a chillul Hashem.