require_auth differs from the auth check of INLINE_ACTION_SENDER but how?

Question

In a contract action I've got the following:

require_auth(from);  // NOTICE THIS LINE...
INLINE_ACTION_SENDER(contract, transfer)(
    N(contract),
    {from, N(active)},  // ... AND THIS LINE.
    {from, to, price, memo});

When I invoke the action I get:

Error 3030002: signatures do not satisfy declared authorizations
Ensure that you have the related private keys inside your wallet and your wallet is unlocked.
Error Details:
transaction declares authority '{"actor":"myaccount","permission":"active"}', but does not have signatures for it.

Notice that require_auth(from) passes but the inline action fails because the authorization {from, N(active)} is not present.

How is that possible? Doesn't require_auth(from) in effect check for the authorization {from, N(active)}?

Methodology

I'm invoking the action as follows:

cleos push action contract action \
  '[ myaccount theiraccount "16 BTC" "DD => PP (C=8)" ]' -p myaccount@active

-p myaccount@active explicitly provides myaccount@active authorization, a fact that I can verify by running, for example, the following...

cleos push action contract action \
  '[ myaccount theiraccount "16 BTC" "DD => PP (C=8)" ]' -p essential@active

...which causes the require_auth(from) to fail as expected...

Error 3030001: missing required authority
Ensure that you have the related authority inside your transaction!;
If you are currently using 'cleos push action' command, try to add the relevant authority using -p option.
Error Details:
missing authority of myaccount

...because I didn't provide -p myaccount@active.

Answer 1

From an answer by Todd Fleming on the EOS Developers Telegram channel:

If user A authorizes an action sent to contract B then that does not imply B can send an action to contract C with A's authorization.

There are two ways to address this. The bad way: have users authorize B to act on their behalf. The good way: contracts can detect when someone transfers tokens to them and act on it.