> I suppose forcing a means to share the source code could have been too restrictive, but the GPL only speaks about the shape of the source code itself (it should be "the preferred form of the work for making modifications to it"), not how it is shared...

With the greatest of respect, you've forgotten what the licenses say.

GPLv2: [0]

  3. You may copy and distribute the Program (or a work based on it, under Section 2) in object code or executable form under the terms of Sections 1 and 2 above provided that you also do one of the following:
  
    a) Accompany it with the complete corresponding machine-readable source code, which must be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or, 
    
    b) Accompany it with a written offer, valid for at least three years, to give any third party, for a charge no more than your cost of physically performing source distribution, a complete machine-readable copy of the corresponding source code, to be distributed under the terms of Sections 1 and 2 above on a medium customarily used for software interchange; or,
    
    ...

GPLv3: [1]

  6. Conveying Non-Source Forms.
  You may convey a covered work in object code form under the terms of sections 4 and 5, provided that you also convey the machine-readable Corresponding Source under the terms of this License, in one of these ways:

    a) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by the Corresponding Source fixed on a durable physical medium customarily used for software interchange.
    
    b) Convey the object code in, or embodied in, a physical product (including a physical distribution medium), accompanied by a written offer [to convey the source code upon request]...  on a durable physical medium customarily used for software interchange, for a price no more than your reasonable cost of physically performing this conveying of source, or (2) access to copy the Corresponding Source from a network server at no charge.
    
    ...
    
    d) Convey the object code by offering access from a designated place (gratis or for a charge), and offer equivalent access to the Corresponding Source in the same way through the same place at no further charge. ...
    
    e) Convey the object code using peer-to-peer transmission, provided you inform other peers where the object code and Corresponding Source of the work are being offered to the general public at no charge under subsection 6d.

This unambiguously speaks about the form in which the source code is shared. If the licenses didn't specify this, folks would be compliant with the letter of the license by shipping you a printout of the source code and everything you need to build it and charging you for both the labor to generate that enormous, heavy-ass printout and shipping and handling to get it to you. [2]

[0] <https://www.gnu.org/licenses/old-licenses/gpl-2.0.html>

[1] <https://www.gnu.org/licenses/gpl-3.0.html>

[2] To downvoters: Don't forget that OCR was decent even back in the 1990s... certainly good enough for a good-quality printout in a fixed-width font to be -strictly speaking- machine-readable, and it has only gotten better as time has wobbled on. If you don't believe my account of the history, go look up how Zimmerman exported copies of PGP back when it was considered an export-controlled munition.

Google has stopped pushing Git tags for some Android source code