How is it legal to place an employee on administrative leave without pay when the employee was not told it was unpaid

Your employer is only required to pay you for the time you are actually working. Since you were suspended, you were not actually working, and therefore your employer is NOT required to compensate you for the 10 days you missed.

The employer is not required to pay you for the suspension period. In Texas, absent an employment contract or a collective bargaining agreement which establish disciplinary procedures, an employer may make, alter, and enforce reasonable rules governing employee workplace conduct. Enforcement of rules and policies may include legal forms of discipline (including reduction of compensation - subject to minimum wage laws - probation, unpaid suspensions, reassignments, and a wide range of other disciplinary measures) and termination. The rules and policies are not required to be rational or efficient. Note, however, that an employer generally cannot engage in disciplinary actions for prohibited discriminatory reasons (such as racial discrimination), or in retaliation for certain protected actions (such as whistle-blowing). Neither are employers are required to treat all employees the same, so long as the disparate treatment is not part of prohibited discrimination or retaliation (as described above). The description of the situation which you have provided does not include the elements of prohibited discrimination. Note also that, with certain limited exceptions (dealing primarily with accrued vacation and PTO), employee handbooks and employer internal policies are very rarely considered by Texas courts to be binding employment contracts. That means the employer can change (or even ignore) its policies at any time, with or without notice.