The proposition that Pāṇḍu's sons not being named after their father Pāṇḍu, (that Kuntī & Mādrī beget from various deities) is wrong. Those five brothers are rightly called Pāṇḍavas, which is derived from the root word Pāṇḍu, and hence meaning - "the Sons of Pāṇḍu". Just like Shri Krishna is called as Vāsudeva (वासुदेव), i.e., son of Vasudeva (वसुदेव) or, Kātyāyanī (कात्यायनी), i.e., daughter of Rishi Kātyāyana, similarly the case is made for the five brothers being called as Pāṇḍavas, i.e., the Sons of Pāṇḍu.
Now, both Dhṛtarāṣṭra & Pāṇḍu were begotten by their mothers (Ambikā & Ambālikā) through Veda Vyāsa - Krishna Dvaipāyana, by the ancient (and dharma approved) practice of Niyoga. And thus, legally and in the sense of Dharma, both Dhṛtarāṣṭra & Pāṇḍu, are sons of Vichitravīrya only and not of Veda Vyāsa, even though Vyāsa maybe their biological father.
Why so? Why Dhṛtarāṣṭra (and Pāṇḍu ) is not called Vyasa's son, but Pāṇḍavas are also referred to by their biological fathers (devtās) names?
As per the standard rules of Niyoga, the progeny conceived, even though not biologically belonging to the husband, is under the eye of dharma, very well approved as legal, as defined in the Manu-Smriti (Chapter 9), and thus, child born by Niyoga is considered as kshetraja (क्षेत्रज) child of the husband-wife.
Further, to mitigate the doubt posed in this question,
You must see that, you are also missing the fact that not only Dhṛtarāṣṭra, but Pāṇḍu is also, a son of Vyasa (in the biological sense). However, the reason for that kind of reference is not in anyway related with assumptions like - " whether if, Dhṛtarāṣṭra was evil or not ".
The Pāṇḍavas being also referenced by their biological father's name, can be thought of in the following two ways:
1. Poetic & Literary sense
In the poetic and literary sense. It's an example of Hero glorification / Hero Worship - i.e, conferring an exalted status to both the Pāṇḍavas and thus, their biological fathers (i.e., the devatās). The Pāṇḍavas are the positive protaganist of the epic. And even if born via Niyoga, but, in this case, it's more of a "Divine origin" Niyoga, since it involves live devtaas as the donors, and thus, they are the direct biological sons of the highly revered and exalted Vedic deities. Therefore, since it involves both the process of Niyoga and the involvement of sky divinities as donors, and further, the use of a highly potent mantra given to Kuntī by Rishi Durvasa. Since, everything about their birth is divine, thus, the Pāṇḍavas are also frequently referred to as by their biological father's name, who are all exalted devtaas. In no way, though, it results in the depreciation of the fact, that they were the dharma approved progeny begotten by Kuntī & Mādrī on the advice and behest of Pāṇḍu himself, and thus rightly, are the sons of the Pāṇḍu.
However, Dhṛtarāṣṭra is not a positive protaganist of the story and Pāṇḍu is more or less, not of significance to the story of Mahābhārata after the prologue (Adi Parva). Thus, they're not to be exalted in the epic in terms of significance compared to the Pāṇḍavas. And hence are unequivocally called as Vichitravīrya's sons only.
2. Dharma, ascetism and the influence of saṃsāric māyā.
In case of Dhṛtarāṣṭra and Pāṇḍu, even though, they are also begotten via Vyāsa - who is himself a highly exalted divine person and also a Vishnu-aṃśa, but ultimately, he's a sage, and an ascetic and is thought of as someone above the clutches of saṃsāric māyā. Thus, Vyāsa is merely performing his karma as per dharma. Thus, Vyāsa, being mostly dispassionate with worldly affairs, is to be taken of as someone, who is just discharging away his duties and hence in no way identifies with his biological sons (as per the rules of Niyoga).
But, Vyāsa is an exception. Almost everyone in the material existence have been known to be influenced by the power of māyā, and thus, deities too, are no exceptions. Thus, the deities (the biological fathers), as we can see throughout the Itihāsa, seem to be very much connected and concerned with the fate of their human sons, especially in case of Indra (Arjuna) & Sūrya (Karṇa), involving the famous "Kavacaha-Kuṇḍala" donation episode. Thus, the Pāṇḍavas are appropriately referred as, at several instances, in the name of their biological father ( i.e., the devatās) due to active involvement by their respective biological father's in helping and protecting them throughout the Mahābhārata.