There are two good reasons to peer only with backbones of
comparable size:
1) technical -- this allows to do hot-potato routing with minimal
resulting delay assymetry
2) economical -- peering at IXP is favouring smaller folks in
cost/benefit game. For a large ISP the percentage of additional
destinations the small ISP offers is smaller, and so is the
benefit of peering. ANd extra peerings are far from "free".
They produce extra paths which need to be processed by routers,
and so translate in shorter lifetime of the equipment -- which
means very real money. It also reduces effectiveness of
aggregation and increases risk of catastrophic failures (large
ISPs spend significant part of engineering resources and practice
rather strict controls on configuration process to ensure sanity
of routing; they also have to _trust_ each other. Most of garbadge
in routing tables comes from small ISPs, and is usually hard to
fix (no 24hr NOC etc).
So the "three IXPs" limitation is specifically designed to ensure
equality of sizes (to a some extent), other provisions of BLIAs
("blia" is a common profanity in Russian, btw 
include 24hr NOC etc.
Thus, far from being anti-competitive restrictions those policies only
serve to level the playing field -- as players who skip investment
in infrastructure, engineering and operations would get unfair advantage
otherwise.
No sane NSP would refuse peering with a new nationwide DS-3 backbone --
providing that it is something more than hot air. The plank is going
to be raised to OC-3 pretty soon, though....
--vadim