The term spam in computing originally referred to excessive Usenet cross-posting and later became a synonym for unwanted broadcast email. While there is no clear distinction between wanted and unwanted email, the messages carry identity, are not fungible, and do not carry payment for processing by the recipient. Bitcoin transactions by comparison are neccessarily anonymous, fungible and carry payment for processing.
While email spam detection is a subjective process, it is necessary due to the lack of payment for processing. This process is facilitated by identity and lack of fungibility. By contrast, due to anonymity and the fungibility objective, there is no test possible for transaction legitimacy, and due to payment there is no need for it. In other words all valid transactions are equally legitimate and this does not subject nodes to denial of service. A proper name for a transaction with a low fee is “low fee transaction.”
High volume submission of redundant transactions is a typical denial of service problem that is independent of the transaction fee and can be carried out by any person, not limited to the spender. Non-redundant transactions that incorporate mutually-conflicting spends are not a denial of service risk, since they are either rejected as invalid or accepted due to a sufficient fee increment.