Its to add more mods but reducing costs of a mod by half. Some, like hte flux rifle, can only fit 6 mods, nonelementals fully ranked up. not including serration.
but with forma, you can add a lot more. take a look at this vid to see what 5 forma can do to the flux rifle. .Warframe Flux Rifle - 5 Forma - YouTube
True power in this game is all about min-maxing. Without Polarization, your options for min-maxing are severely limited. You need to squeeze out those last few points to obtain maximum power from a frame or weapon.
Are you a Mastery Rank 11 or 22? If so then no, it is not a bug. The most recent update brought in a new game mechanic for weapons and Frames. Any new or Forma-ed weapons/Warframes will have a starting mod capacity equal to your Mastery Rank.
If your'e a Mastery Rank 22 then any new gear you buy or Forma will have a starting mod capacity of 22 at unranked.
If you're a Mastery Rank 11 then any new gear you buy or Forma will have a starting mod capacity of 11 at unranked, and 22 if an Orokin Reactor/Catalyst is installed on the gear.
Basically: Starting Mod Capacity = Mastery Rank (x2 if Orokin Reactor/Catalyst installed). You should note that mod capacity on gear will still have the same cap though, you will not gain an extra 11 or 22 capacity.