Hi,
When it comes to BPO companies, they are more informal, and they want their employees to be very social. That is the reason why you can see most employees wearing sleeveless tops and jeans not only during weekends but also on regular days.
The second question is whether a company can impose a dress code by force?
Normally, a company can establish rules, regulations, and HR policies regarding dress code and other matters. If the company believes it should enforce restrictions, they can do so, or else they can allow more freedom. Since many MNC companies are entering the Indian market, they may not pay much attention to dress codes and other minor issues.
Your third question is whether it infringes on individual liberty or not?
No, it becomes more casual. If you wish, you can also wear sleeveless tops and jeans.
When working in BPO or IT-related companies, the environment is casual, and activities like chatting, discussing, and going out are common. The mentioned behaviors have been observed, and some information was shared by my friends. While you are discussing individual liberty, there are many other things beyond dress code that may be beyond a company's control, potentially leading to immediate termination for the employee concerned.
Your fourth question, "whether it is legally tenable?"
No, it is not.
I hope this helps. Let me know if you have any other questions.