As far as I know, most regs have some sort of out that says "unless directed by ATC," and ATC has two unbreakable rules which limit much of what they are allowed to do:
- thou shall not vector airplanes into obstacles, terrain, unflyable weather or atmospheric conditions, restricted (in the lay sense of the term) airspace, or other aircraft
- thou shall not deviate from aircraft separation
Other than that, when it comes to clearances, ATC has a fair amount of latitude to give you what they want, or what you ask for.
The balance of responsbility between ATC and the pilot in command, is covered by 14 CFR 91.123:
§ 91.123 Compliance with ATC clearances and instructions.
(a) When an ATC clearance has been obtained, no pilot in command may
deviate from that clearance unless an amended clearance is obtained,
an emergency exists, or the deviation is in response to a traffic
alert and collision avoidance system resolution advisory. However,
except in Class A airspace, a pilot may cancel an IFR flight plan if
the operation is being conducted in VFR weather conditions. When a
pilot is uncertain of an ATC clearance, that pilot shall immediately
request clarification from ATC.
(b) Except in an emergency, no person may operate an aircraft contrary
to an ATC instruction in an area in which air traffic control is
(c) Each pilot in command who, in an emergency, or in response to a
traffic alert and collision avoidance system resolution advisory,
deviates from an ATC clearance or instruction shall notify ATC of that
deviation as soon as possible.
(d) Each pilot in command who (though not deviating from a rule of
this subpart) is given priority by ATC in an emergency, shall submit a
detailed report of that emergency within 48 hours to the manager of
that ATC facility, if requested by ATC.
(e) Unless otherwise authorized by ATC, no person operating an
aircraft may operate that aircraft according to any clearance or
instruction that has been issued to the pilot of another aircraft for
radar air traffic control purposes.