Why do you think using public Wi-Fi is wrong?

Agree with all the answers here for security reason don't use public wifi, especially for the online transaction like banking and shopping online.

And for your safety use trusted VPN service. VPN will secure your data from the network. 

For security reason because it might be noticed during doing any activity that can help the hacker to hack you and steal your information and use it against you later  

Because your personal inform is prone for hacking. ( specially your password) for all accounts.

Using public Wi-Fi is may cause of stealing your information, more vulnerable to threat of being theft of personal information  specially while you do transactions.

With the use of public Wi-Fi, anyone can access your mobile via hacker software

Therefore it is recommended not to enter such networks unless known source

I think there is no wrong if using private VPN also don’t try to use online transaction like banking and shopping online

Some ISP companies use the login information like emails and phone numbers  of wi-fi for marketing of some products 

There are a few big problems with using a public Wi-Fi network. The open nature of the network allows for snooping, the network could be full of compromised machines, or  — most worryingly — the hotspot itself could be malicious

Using public Wi-Fi make your phone become more vulnerable to threat of being theft of personal information like Credit card password or transaction passwords etc   

it's really easy to be hacked or at least stoled

Advanced access to the source gateway could cause really security issues, for example when you chatting with any application that not encrypt the messages and it do warn you that messages are not encrypted, each message sent to router to be delivered through the gateway, here some sort of hacking could interrupt the messages. Also, sites visiting are some sort of private matter, you don't need someone to blame you for opening a site or have this information related.