I think that's what happens by design when you restore a full/log backup on SQL Server.
I did some quick tests on SQL Server 2000, and the buffer cache drops all cached data related to the database whose full/log backup you are now restoring. Makes sense since it does not know if the data it holds in the cache is now consistent with that in the database.
I can confirm that SQL Backup itself does not explicitly clear the cache during a restore.
SQL Backup Consultant Developer
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- SQL Backup goodies under the hood, updated for version 8