You are right zeshukhan sahab its really mind boggling how the minds of certain educated people also get corrupted in the name of modernization and liberalism. Whereas Iqbal has already said:
Khaas hey tarkeeb mein Qoum e Rasool e Hashmi
I am failed to understand what is wrong in preventing the Qadyanis from using the word Islam? Was Muselma Kazzab not killed? As you must know, during the last part of the life of Prophet Muhammed (PBUH), Uswad Ansi and Muselma Kazzab made false claims to Prophethood. Uswad Ansi was killed by Hazrat Feroze Dailmi Radi Allahu unhu in Yemen and after hearing this news, Muhammed (PBUH) had said "Last night, Uswad Ansi was killed he was killed by a blessed person from the house of the blessed. He (Sallallahu Alayhi Wasallam) was asked about the person who did this. He (Sallallahu Alayhi Wasallam) said that Feroz did this Feroz has succeeded (in hereafter).
After Muhammed (PBUH) had passed away, Ameer ul Momineen Hazrat Abu Bakar ordered the Muslims troops to find and kill the Muselma Kazaab. Muselma Kazzab was killed along with some tens of thousands (about 28,000) of his followers in the battlefield of Yamama by the Muslims forces under the command of Khalid bin Waleed Radi Allahu unhu. About 1,200 Muslims also got martyred and this was the largest number of Muslims martyred in the last 10 years. Please note that the total number of Companions martyred in Jehad during the ten years of Prophets (Sallallahu Alayhi Wasallam) life in Medina is 259, whereas those martyred defending the cause of Finality of Prophethood against the apostates amount to 1200 included amongst them are 70 Ahl e Badr (companions who took part in the battle of Badr), and 700 reciters and memorizers of Quran. This includes the Imam (Prayer Leader) of Quba Mosque, one of the four notable reciters (Salim Mola Huzaifa Radi Allahu unhu), elder bother of Hazrat Umer Radi Allahu unhu (Zaid bin Khattab Radi Allahu unhu), spokesman of Prophet Sallallahu Alahyi Wasallam (Sabit bin Qais bin Shammas Ansari Radi Allahu unhu) and prominent companions, Hazrat Tufail bin Umro and Huzaifa bin Yaman Radi-Allahu unhum.
This was the importance of the cause of Finality of Prophethood.