Government is not allowed to collect and store data on citizens without a warrant.

No such prohibition applies to the private sector, although data privacy laws are starting to become more popular globally, so this may change - although if they don't give citizens the right to sue corporations who unlawfully collect and store personal data, they'll be ignored. See Americans with Disability Act.

Hence, governments simply avoid the need to get a warrant by purchasing the data from the private sector.

Incidentally, in "Permanent Record" Snowden said that ensuring the data was kept in the private sector was a kind of victory, but in reality that's meaningless. The merger of government and corporate systems is well under way, particularly in the military-industrial-tech-spying world.