It's not about allowed or not allowed. Everyone can visit the temple throughout the year but door of temple remain open only between a specified time generally (April - Nov).
Treker and begpacker can visit there throughout the year these people generally came for treking and photography. However purpose of pilgrims is to have Darshan of God Only.
I hope you got your answer.
Trekking doesn't have to be a pilgrimage thing. People mostly go to Chandrashila for the views in this case, Its just a sport activity if u see it that way