You are conflating two separate issues.
The law of return, I believe, was set up to mirror Hitler's Jewish laws. Since Nazi laws defined a Jew as someone with one or more Jewish grandparents (the law was more narrow within Germany itself), Israel decided to give a safe haven to anyone who fit that definition -- even if that person is not religiously Jewish. For example, there are many Russian Orthodox Christians who have moved to Israel under the Law of Return and, once there, have set up Russian Orthodox churches where they worship freely. The law of return has nothing to do with religious practice or belief, or even with Zionist or anti-Zionist belief; it was enacted to enable Israel to be a place of sanctuary for those persecuted because of Jewish ancestry -- even those who may be opposed to the existence of Israel (in my opinion, a remarkable act of grace).
I do not know whether, as you say, people who convert to Judaism in non-Orthodox conversions are permitted to immigrate to Israel under the law of return. If this is true, there is logic behind this policy. Orthodox Judaism adheres to the body of rabbinic Jewish laws established over the past 2000 years (it really does, too -- Rabbis making a determination under Jewish law may refer to tens of sources from all over the world and writing over the entire period from before compilation of the Talmud to the present day). Conservative Judaism allows for much more liberal interpretations of Jewish law, and Reform Judaism does not consider earlier rulings of Jewish law to be binding.
The Orthodox conversion process is lengthy and difficult, and an Orthodox convert is universally accepted as a Jew. On the other hand, Reform and (to a lesser extent) Conservative conversions may be significantly less rigorous, and those converted by Reform and Conservative rabbis are not universally accepted as Jews.
From Israel's point of view, there would be no reason for Israel to allow people whom it does not consider Jewish to move to Israel under the law of return.
I should add that the law of return is not the only way for people to become citizens of Israel; it is the simplest, most non-bureaucratic means to do so. Further, it is not necessary to be a citizen of Israel in order to live there.
I have friends that are converts that made Aliya. They were given exactly the same benefits and rights as born Jews. The Israeli Government paid their way to come to Israel, paid to ship their furniture, and set them up in housing. There was no difference given to them. And by the way, they were never asked if they believed in Jesus!
Putting Hitler ahead of Moses--very bad idea.
As a non-Jew I attended a Conservative synagogue for awhile. There were two rabbis, the older of which always wanted to consider me in a minyan while the younger one always balked.
It was frequently awkward as the shul always struggled for a Saturday minyan.
As I said, I attended for awhile....