The system of incentives is all screwed up. The glut of postdocs is a boon for everyone involved, except the postdocs themselves.
Postdocs are very highly qualified and underpaid scientists, who can on a project without almost any supervision, and who are very motivated so will work really hard with little pay. For a group leader, it's a no brainer to hire as many of them as he can.
Imagine how much the cost of science would increase if instead of being able to hire 3 or 4 postdocs at 50k each, you actually had to hire someone with a similar skillset at market pay.
Available funding is constrained complicated ways, and the whole incentive system is a mess.
Departments/PI's can't hire for the positions they would choose, so they hire postdocs for very short terms (often 2 years or less) because they can. Candidates can't get the positions they want (e.g. TT positions) so they take post-docs until they age out or find/settle on something. Everyone understands that there may not be a classic TT position for everyone interested, but the system has so far mostly failed to come up with any sort of viable alternative for long term employment. Not that TT is what it used to be, either.
As a result, continuity in labs is very difficult to maintain as the senior people rotate out too often. Couple this with the growing tendency for PI's to spend more time fund raising than doing research and you have a lot of lost and wasted effort.
Only in that the suppressed wages of engineers at Apple, Google, Adobe, etc., were the "market rate" through collusion. Or that $0 is the market rate of NCAA athletes who earn billions for corporate profiteers.
Since the private sector/industry work is competitive, it is considered market rate. The reason that post-docs are not considered market rate is that the institutions do not compete with each other, and the rate is determined without concern of the applicants.
Honestly, it matters more whether their skills are being put to good use. 50k is adequate salary for anyone on the planet, and nobody complaining about that salary deserves that much sympathy. Complaining about grunt work when you could be better used is fine. Complaining about inequity in the system because others get paid much more is fine.
Postdocs are very highly qualified and underpaid scientists, who can on a project without almost any supervision, and who are very motivated so will work really hard with little pay. For a group leader, it's a no brainer to hire as many of them as he can.
Imagine how much the cost of science would increase if instead of being able to hire 3 or 4 postdocs at 50k each, you actually had to hire someone with a similar skillset at market pay.