Why does Apple allows force update to newer version for few apps?

Are you sure that's an actual App Store update and not the app simply updating its content from one of their servers? At any rate, there is no mechanism in that automatically forces users to update their apps. You'd have to write code into your app where it checks to see if there is an update available then refuses to run if there is an update (you may run the risk of having your app rejected for removing functionality).

It seems fairly common with client/server applications...which, I guess, means most of them now.

If something changes in the server interface, it may be necessary to update the client to match or else old clients simply fail.

I don't see that it's against any guideline.

Yes this is actual App Store update, it takes me to App Store if i click update button and there is no other button like cancel or update later. In simple words I am unable to use the application and there is only one option to update application from App Store. Whenever i open application named farmville2 It shows me only one message with update button and no other option. Application app store url is as follow: https://itunes.apple.com/us/app/farmville-2-country-escape/id824318267?mt=8

Agreed with you, but this is not my question, I am wondering can we force user with only one option on application launch to either update application or don’t use it. Because in other words reference application is forcing user to update which I believe is against Apple rules. Please correct me if I am wrong this means I can force user with only one option on application launch to update app or just don’t use it.

It will be great if you can share the answer or findings in this regards.

Good question.