We've chosen to limit access to these features as a means of protecting our users from those who abuse Steam for purposes such as spamming and phishing.
Malicious users often operate in the community on accounts which have not spent any money, reducing the individual risk of performing the actions they do. One of the best pieces of information we can compare between regular users and malicious users are their spending habits as typically the accounts being used have no investment in their longevity. Due to this being a common scenario we have decided to restrict certain community features until an account has met or exceeded $5.00 USD in Steam.