Serious question.
Is there some particular part of the reading list that made you think that Routledge had a disproportionate representation? I've just checked all the books in the Europe reading list that had links (i.e. up to and including the Middle Ages) and here are the publishers represented:
That looks like a very reasonable cross-section of publishers to me (even if I admit I was startled to see HarperCollins and Random House rank quite so highly). Routledge is a respected academic publisher, so fifth equal seems a pretty reasonable place to see them. Is there another section that caused you concern?
It's certainly not true for the medieval Europe section - none of them are. Where are you looking?
And as a 'serious' question (which means it probably breaks the 20 year rule) it comes down to two factors: does that press or imprint have a particular interest in publishing history; and do they do it cheaply and respectably?
Certain presses, such as Routledge, Palgrave Macmillan and Penguin, pick up popular but academically solid books (eg. David Carpenter, Struggle for Mastery, 2004).
The books are meant for introductions to the subject. These types of introductions might be printed by Cambridge or Oxford Uni. Presses but they are just as likely to have been commissioned or picked up by popular imprints such as Routledge History.
There is nothing inherently wrong or sinister in recommended books being published by non-university presses. These are idiosyncratic choices made by the flared users. I could go and put Huw Pryce, Acts of the Welsh Rulers, 1120-1283, ed. Huw Pryce with assistance from Charles Insley, University of Wales Press, Cardiff, 2005) on there. Only someone with a vested interest in Welsh history (and technical ability - large chunks of it are medieval Latin - although usefully summarised by Pryce) would be able to make good use of it. If they have those skills and the interest then they probably know about it already!
If you are concerned or curious about the academic credentials of any of the books on there (skepticism towards sources is always highly commendable!) then find academic reviews of the books on JSTOR or http://www.history.ac.uk/reviews/ which should help you separate the wheat from the chaff.
May I also add that even a publication by a reputable press does not mean a reputable book. I will not cite the author or reviewer here but here is the concluding comment of a review of a recently published Cambridge University Press monograph:
This is a book which lacks the breadth to which it pretends, and the scholarly approach one expects from such an imprint. It can only be a matter for raised eyebrows that its incapacitating weaknesses were not picked up before it was launched on an unsuspecting public.
I can only assume you're making an allegation of the mod team receiving money to advertise Routledge books. Other people did the analysis above on how Routledge isn't really over-represented, but I think we should add this:
The mod team does not make any money, in any form, for moderating this subreddit, apart from the occasional gilded comment. We've turned down opportunities to monetize the subreddit before--it's entirely volunteer, and will remain that way for the foreseeable future.