In short, no!
It depends on which company built the network, but there must have been a reason.
If the cap is not put back on or is missing, the next time that the tee is used invariably it will be blocked and will need clearing it can even lead to blockages in the main duct. And ntl will more than likely be issued a notice by the local authority highways inspector (for which they are charged) and given a set time to rectify it. If when the LA check again it is still not done, they will issue the notice again and charge again. It leads to Personal Injury Claims as well - trip or fall? not your fault? then sue!