Perhaps not a complete answer, but to provide several considerations:
- people simply could be ordered or paid to join the bloc party, the leaders of such a party could be even undercover members of secret services.
- people could see it as a legal way to air their grievances and divergences with the governing party, and even obtain some concessions - without directly coming to power
- people could be simply naïve
All these typically imply that the government (or the ruling party) does not have the full control and/or knowledge of possible opposition among the population, and tries to either let the people let the steam out, render the oppositional forces less powerful, harness them into collaboration or at least discover them. Indeed, compared to the iron fist with which the Soviet population was ruled, the Communist oppression in the rest of eastern Europe was relatively mild (I stress the world relatively, and I am not sure this applies to East Germany, whose government Gorbachev characterized as Stalinists just before the fall of the wall.)
One famous example is Zubatovschina - creation of police-sponsored trade-unions in Russia in the early XX-th century. Up to then the Czar's government treated any kind of assembly, association, organization, etc. as a political threat and used repressive measures to prohibit them (arrests, exiles, censorship, etc.) However, it was well understood that such measures did not reduce the underlying grievances, which could have eventually led to a spontaneous revolt - in fact the situation in Russia was rather similar to that in France a century earlier, in the years just before the French revolution. It was in this conditions that the government sanctioned police-sponsored trade unions - their activities were supposed to be limited to the economic disputes with the employers, that is not involving any political demands (like universal suffrage, transforming absolute monarchy into a constitutional one, etc.) Their open collaboration with police guaranteed that no such subversive activities would take place. (Some referred to this as "police socialism", since in a sense the government was supporting the workers in the disputes with the employers.)
In practice the idea didn't work as well as expected, due to a Charismatic union leader Father Georgy Gapon organizing a many-thousands-strong procession towards the Winter Palace (Czar's residence) with petition of mild but political nature (the petition followed a series of strikes). There exist contradictory accounts of what exactly happened: either poorly prepared police "accidentally" fired at the peaceful demonstrators or the crowd was too big and became uncontrolled, but several hundreds people were killed and the Russian revolution of 1905 had began. It would end about a year later, after thousands more deaths and major concessions by the Autocracy - notably the creation of the first Russian parliament, Duma - although mainly with consultative functions:
The October Manifesto, aside from granting the population the freedom of speech and assembly, proclaimed that no law would be passed without examination and approval by the Imperial Duma. The Manifesto also extended the suffrage to universal proportions, allowing for greater participation in the Duma, though the electoral law in 11 December still excluded women. Nevertheless, the tsar retained the power of veto.
In the photo: Father Gapon and General Fullon, the prefect of St. Petersbourg police, in a union meeting (image source.)