There are exceptions..
Any airline site should have "Required Travel Documents" section that will give you all the information. Heres what nwa.com has listed for your case. Seems like since your parents have a "valid" US visa, they wont need a transit visa.
/ 08SEP06 / 1822 UTC
National INDIA (IN) /Residence QATAR (QA)
Embarkation U.S.A. (US)
Transit UNITED KINGDOM (GREAT BRITAIN) (GB)
Destination QATAR (QA)
ALSO CHECK DESTINATION INFORMATION BELOW
UNITED KINGDOM (GREAT BRITAIN) (GB
Visa required, even in direct airside transit.
Exemptions apply when holding visas for Australia, Canada,
New Zealand, U.S.A. and E.E.A. countries (TIRGL/EEA )and
TWOV is then permitted. For information & conditions on:
- exemptions, see: For details, click here ; and
- TWOV facility, see: For details, click here .
Type of transit visa required depends on type of journey:
- if passenger is in transit with same-day, same-airport
transfer, "A-Direct Airside Transit" visa (DATV) is required.
This visa must be obtained prior to arrival in the UK and
will allow the passenger to transit between terminals at the
same UK airport as the passenger remains airside;
- if passenger is in transit and must change airport in the
U.K., or has an overnight stop in U.K., a valid U.K. visa
with any endorsement other than "Direct Airside Transit
Visa" is required;
- if passenger is in transit to the Channel Isl., Isle of
Man, or Ireland (Rep. of), a valid U.K. visa with any
endorsement other than "Direct Airside Transit Visa" or
"Visitor in Transit Visa" is required.
Minors:
if children are registered in parent's or guardian's passport:
- they may not travel without holder of passport; and
- visa in passport (if required) must indicate it is also valid
for the child(ren).
Non-compliance with transit requirements (incl. forged
documents) results in fines for carrier of GBP 2,000.- per
passenger and either deportation or indefinite delay for
passenger at point of transit (detention costs at carrier's
expense).
Minors:
if children are registered in parent's or guardian's passport:
- they may not travel without holder of passport; and
- visa in passport (if required) must indicate it is also valid
for the child(ren).
Non-compliance with transit requirements (incl. forged
documents) results in fines for carrier of GBP 2,000.- per
passenger and either deportation or indefinite delay for
passenger at point of transit (detention costs at carrier's
expense).
Flights from/to United Kingdom to/from Channel Islands, Ireland
(Rep. of) and the Isle of Man are domestic flights therefore
there is no immigration control.
