I've heard of this too, some people praise such apps highly.
As Mr. Whisonant said, you have to be mindful of the risks involved, however i would be less worried about legal implications of someone using your connection, but more worried about someone giving your phone a virus, a trojan or some other thing that could compromise your credentials and/or identity.
Why i think legals are PROBABLY&IMHO not an issue - cafes provide internet connection to hundreds of people per day, and inevitably some use it for illicit purposes. Which means there has to be some waiver of liability in the applicable laws.
I forgot about which apps were used, but i heard people made up to $100 per day during specific events, by either sharing their bandwidth or by providing a charging station - via their personal phones and powerbanks.
IMHO the level of risk involved in both options is just stupid. But it goes to show how many ways there are to earn some side money - if you are crafty and a bit bold.