[zaarn]

Wikimedia has a User-Agent policy which is being violated here. Hence this is the property owner putting up a sign that says "risk of injury", so if you walk in and injure yourself, you only have to blame yourself for being negligent.

[h_anna_h]

The policy is for how wikipedia will act when encountering clients with certain user-agent headers, not a rule for the clients.

[zaarn]

It's a policy how wikimedia acts when clients lack a user agent header, it's therefore effectively a rule for clients as without a proper UA header, they may be blocked indefinitely.