Can the service providing the bitcoin wallet know how much value has the user loaded into his wallet? Say for example I run an ecommerce website which mandates users to have a minimum balance of 1 bitcoin in their wallets to access the platform. In the event the balance in his wallet falls below 1 bitcoin he should not be able to access the platform and he should see a message on his screen saying "Please load to minimum balance". 

So, the question is, can I, or the Bitcoin wallet provider I use, read the bitcoin balance of the users so as to fulfil the above business requirement?
All Bitcoin transactions are publicly available. This is fundamentally why Bitcoin can work as a truly decentralized currency. Bitcoin is different from the traditional banking system specifically because anybody can see and validate everyone's transactions. If only a few could access the information about people's balance, then it wouldn't be fully decentralized, it would be just like traditional banking.

So yes, you can calculate the balance of any Bitcoin address in the network.