I'll not discuss other points ( regarding the knowledge of Mirza Ghulam Ahmed Qadyan and Illam Iqbal as it may divert the topic from what you have asked)
So the answer of the question is
Pakistan is the country, founded for the Muslims, the foundation of Pakistan is Islam and not any other religion. As a result you can see, Muslims are in Majority. Pakistan have never claimed to be a Secular state, so preaching another religion is out of Question. It is like, can anyone preach Islam in Israel??? Now it still can be considered, but the way Qadyanies preach their religion are awkward. for example, they start with , "we are muslims..." this is where normal Pakistani muslim get offended, as how come, they be Muslims, they are Qadyanies, why don't like qadyanies start their preaching with, "we are Qadyanies.." and then tell everyone about their religion and Mirza Ghulam Ahmed, but for some obvious reason they are not doing it... why??? can you tell me this!!!
Qadyaniat if it is true and divine religion, then it shouldn't be hiding itself, it should be open to every one, .. but as i said, the Qadyanies for some reason do not practice this, they work under the veil of Islam...
here in this forum, Qadyanies are insisting that they should be called muslims, why??? we muslims are those who believe that Prophet Mohammad (SAAW) was the last prophet and anyone after Him, claiming prophet-hood is Dajaal, a liar, an impostor...So why don't you select and adopt the name of Ahmadi or Qadyani for your self.. why insist to be Muslim??? and then preach qadyaniyat...
Anyway, little note for you, Qadyanies, are not in that big number, nor they are only one who follow some false prophet and survived, Beha'i are example of the same.. anyway there are two more guys in todays world, 1) Riaz Ahmed Gohar Shahi 2) some one called Jasim, from former Yugosolavia each claimed to be Mahdi.... so what do you think of them??? is the claim true!!!
PS: I do not believe in Any Imam Mehdi, nor believe in such personality will effect my belief as a Muslim
Can I say something (a little irrelevant to topic)
There is no faith or sect or people who practice something called "Qadyaniyat".
If you want to relate us to some faith or sect then it is "Ahmadiyyat".
I would rather take it as offensive word and point it out for the attention of Moderators.
(P.S - Just for records Jamaat Ahmadiyya consist of 200 Million strong followers.)