Why I am able to Telnet to VIP when pool members are down ??

That's indeed right for standard virtuals with a TCP profile attached to it, but the BigIP will nevertheless issue a TCP reset immediately after the established 3WHS, so the connection should not stay "up". If you have a fastL4 virtual, the BigIP will instead reply with a TCP Reset to the first SYN packet of the client, so the connection will not even establish. See https://support.f5.com/csp/article/K9812 for details:

When all pool members are unavailable due to being disabled, forced offline, or down, the BIG-IP RST behavior varies slightly depending on the virtual server type. If the virtual server references a TCP profile (Standard virtual server type), the system allows the three-way TCP handshake to complete before sending the TCP RST to the client. If the virtual server references a FastL4 profile, the system sends a TCP RST packet in response to a connection attempt. The TCP RST packet is sent on the client side of the connection, and the source IP address of the reset is the relevant virtual server IP address.